This paper considers stock price manipulation by a dynamic informed trader. We provide a simple proof of the existence of manipulation in a market in a standard sequential trade model. We also give the lower bound of the number of trading periods for the existence of manipulation in equilibrium and show that if the number of trading periods is larger than that lower bound, every equilibrium involves stock price manipulation. Irrespective of the prior of the market maker, if the informed trading probability is high enough, every equilibrium involves manipulation.