As a very important issue in behavioral finance,Overreaction Hypothesis (ORH) stands in contra- diction to the Efficient Market Hypothesis (EMH).This paper employed the Reversal Coefficient,which could be used to compare efficiency in different security markets.The results illustrated that the horizon of overreaction was the longest in China' s security market whereas the shortest in US' s and UK' s markets and the second longest in Japan' s market.