(Hillier, Ross and Westerfield, 2010)And they use CAPM to compare the expected stock return estimated by CAPM to buy the cheapest stock. The beta coefficient is the most difficult estimated value in the CAPM and it is the most important value in the CAPM used to measure the market risk of an asset. Beta is equal to 1 when the percentage change in stock price and market price is the same. When the beta is greater than 1. For example, the beta is 2.0 means that when the market price increases by 10%, the stock price will increase by 20% and vice versa. Therefore, investors can invest their money in stocks with lower beta when the market price decreases and invest in stocks with beta greater than 1 when the market price increases. This model is widely used in investment. Beta is calculated from historical data and data length is also very important for beta variance. The longer the time, the more stable the variance. However, if the duration is too long, changes in business operations, market, technology, competitive strength and market characteristics will affect the beta value. Additionally, many popular index funds offer investors the opportunity to diversify at low cost. Limitation of CAPM when estimating the investment potential of a company's stock from the investors' perspective. The CAPM assumptions are the limit of this system