Taking Markowiz's portfolio theory as the basic theory, this paper selects 20 stocks from FTSE TWSE Taiwan 50 Index and makes an empirical test using the monthly return and variance data of Taiwan stock market from 2011 to 2014. Finally, the relationship between portfolio size and portfolio risk is obtained, and it is found that when the portfolio size reaches 12, the effect of risk diversification that continues to increase the portfolio size will no longer be obvious.