In market economy, there are four types of markets: perfect competition, monopolistic competition, oligopoly, monopoly. The main differences among them are the ability to set price, barrier to enter and exit the market, numbers of companies. To study innovation’s efficiency in these markets, it is necessary to understand their special characteristics. To simplify the problem, when patent is employed, only the innovation company has the access to this new technology. When it does not exist, every company in the market can use the new technology. In perfect competition market, there are no barrier to enter or exit and lots of companies producing identical products, so no company can set the price. Because there is no barrier, companies that can earn profit will enter the market, which decreases the price. Eventually, all companies’ marginal cost, average cost and marginal benefit is equal to the price, average benefit. In other words, companies in perfect competition market earn zero economic profit. Social welfare is always maximum in this type of markets. In this case, when one company discovers new production technology, other companies will follow immediately. Lower cost causes higher supply, which makes the price decrease and equal to the average cost eventually, leaving every company having zero economic profit, including the first company discovered the new technology, so there is no incentive for any company to spend resource on innovation. However, consumers’ welfare would increase because of lower price. When patent is employed, one company can produce products in a lower price and earn certain economic profit, but can hardly make an influence on the market because there are too many suppliers. Thus, in perfect competition market, patent is a good way to provide incentives for innovations. In monopolistic competition market, there are lots of companies selling slightly different products. The difference among products enables one company to increase the price over in a limited range, so monopolistic competition market is inefficient. In this type of markets, there are two types of innovations: technology and product. The former one reduces the cost and has the same consequence as that in perfect competition market. The latter one, product innovation, makes the product more special, giving the company more market power. However, without patent, product innovation will be copied easily, making the original product less special and canceling out the market power gained by the original company. Since there is no economic benefit, there is no incentive for any company in the market to innovate. When patent is employed, products’ difference is kept and gives the company more market power since there is consumer preference in monopolistic competition market. This increase of market power is not as negligible as that in perfect competition market, so the market becomes less efficient when the company with patent increases the price. In oligopoly market, there are only a few companies with great market power, so all of them can set the price. In this market, companies make decision based on both output and price effects. Output effect means when price is higher than marginal cost, companies can increase profit by increase its output. Price effect means when a company increases its output, the market price goes down, causing less profit for the company. When output effect is more impactful than price effect, companies will increase sales. When price effect is more impactful than output effect, companies will decrease sales. Oligopoly market can be inefficient without restrictions. Regarding innovations, there is still no incentives without the presence of patents. With patent, innovation company will gain market power that is huge enough to cause inefficiency and even to force other companies to exit the market. Thus, patent in oligopoly market will cause negative impact on society, which should be limited. The last type of markets if monopoly. In monopoly market, there is only one company, so patent is necessary. When this company innovates and decreases its production cost, it will tend to increase its output to maximize profit, which enlarges consumers’ welfare. However, this increase is not as much as that in perfect competition market, so innovation in monopoly market is still inefficient.