投資人在臺灣股票市場該投資多少家股票才可算是充分分散投資風險，似乎是被國內學者所忽略的問題。本文重新檢視此一問題，結果發現投資我國股票市場所須投資家數與降低可分散風險的關係不同於美國市場。若以Bird及Tippett的標準，認為應該投資22家股票。然而我們模擬臺灣股票市場的結果則是，以三個月、六個月及一年的投資風險測度來看，投資人僅需投資4-5家左右的股票即足夠了。因此雖然股票投資組合的變異數與股票家數間有著恆等的關係，但是不同的市場結構會有不同的投資策略。若要消弭部份可分散風險，投資於我國股票市場所需的股票家數就不必如國外學者的建議，而可以較少的家數就能完成充分分散風險的功能了。

　　　　　How many stocks should be bought in Taiwan's stock market to construct a well-diversified portfolio﹖Such a simple issue has been ignored by Taiwan's financial scholars for a long time. We examine the above issue and find that the number of stocks an investor needs to buy is far less than the number suggested by the traditional wisdom. Using 3 month, 6 month and 1 year as the investment horizon, this note finds that an investor needs only 4-5 stocks to reach the risk level specified by Bird and Tippett. Although there exists an identity relationship between portfolio variance and the number of stocks, an investor still needs to develop different investment strategies in different market structures. The number of Taiwan's stocks constitute a well-diversified portfolio is far less than the traditional suggestion is a living proof.