In many cases, the stock prices had great fluctuations, without any change in specific and reliable information existed in the stock market. These defects in the standard financial theory make a field of study based on psychological theories called behavioral finance. Herd behavior as a part of this domain has special importance. In herd behavior, decision makers rely on other investors' decisions and follow them without any logical thinking. As a result, such decisions cause unconditional subordination of the other investors' behaviors and a large group of investors rush to opt the same stocks to purchase or sell without publishing any information simultaneously. In this research, Herding behavior in Tehran Stock Exchange is investigated. The research methodology is based on decrease of deviation of rate of return in stress periods compared with other periods. The results show that there is no herding behavior in four selected portfolio that are examined. There are similar results for tension periods with return decreasing and tension periods by an increase in returns too. Thus, investors had reasonable decision making and rational behavior. On the other hand, return standard deviation increases at the periods of stress similar with CAPM anticipation.