本研究主要探討投資人的積極程度以及市場的樂觀程度是否會影響動能策略之獲利能力。本研究利用1973至2013年間美國個股進行實證研究，結果驗證了動能策略於樣本期間能有顯著的獲利。進一步的實證結果顯示，規模較小且交易量成長率較低的公司存在極短期(一個月內)反轉的現象。此外，在市場樂觀期間(較多的首次公開發行的公司家數、較高的消費者信心指數或較低的恐慌指數)動能策略之獲利能力較佳且顯著。因此，我們建議投資人能在市場樂觀期間對規模較小的公司進行動能策略，將可得到較高的預期超額報酬。 / The main purpose of this study is to investigate whether the activism of investors and the sentiment of the market can affect the profitability of the momentum strategy. Using individual firms during 1973 to 2013 as the sample, this study reexamines and confirms the profitability of the momentum strategy. The further empirical result shows that firms with smaller size and lower growth rate of trading volume exhibit a very short-term (within one month) reversal effect. In addition, during the optimistic period (years which have more firms conducting initial public offerings, higher consumer confidence index, or lower VIX), the profitability of the momentum strategy is significantly higher than that during the passive period. Therefore, a suggested trading strategy applying momentum strategy to small firms during the high sentiment period may yield a superior performance.