Herd behavior, a situation in which a group of individuals or institutions react coherently without any coordination between them, is said to be relatively more frequent in Korean financial markets. This paper introduces many examples of herd behavior in Korean financial markets and tries to provide a theoretical explanation about them. In particular, to deal with the issue of "herding for certain themes", this paper uses the model in Froot, Scharfstein & Stein (1992). Adding positive feedback traders to the model, this paper finds that positive feedback traders can weaken the herd behavior between speculators' research activities if the information is symmetric to the speculators and the market maker. However, the presence of the positive feedback traders can strengthen the herd behavior between speculators' research activities, when the information about the positive feedback traders is known to the speculators but not to the market maker. This effect arises from the strategic complementarity between speculators when their time horizons are short. From this analysis, this paper suggests various policy options to enlarge institutional investors' time horizon and to enhance the research abilities of financial institutions.