Hafnium dioxide ($HfO_2$) thin film as a gate dielectric for organic thin film transistors is prepared by plasma enhanced atomic layer deposition (PEALD). Mostly crystalline of $HfO_2$ film can be obtained with oxygen plasma and with water at relatively low temperature of $200^{circ}C$. $HfO_2$ was deposited as a uniform rate of $1.2;A^{circ}$/cycle. The pentacene TFT was prepared by thermal evaporation method with hafnium dioxide as a gate dielectric. The electrical properties of the OTFT were characterized.