The ability of ALD to deposit thin films with great uniformity over large area and low temperature deposition make it viable technique for flexible flat panel display. Recently, transparent thin film transistor is being considered as important component for next generation display device. For this, all layers including active channel layer, gate insulator, source, drain, and gate should be made of transparent materials. For example, ZnO is widely studied as a promising material for active channel layer of TTFT. In addition, high k materials including Al2O3, and transparent conducting oxide such as Al-doped ZnO can be used for gate insulator and electrode, respectively. In this study, we investigated the ALD process for ZnO, Al 2O3, and Al-doped ZnO and fabricated the TTFT from all ALD based thin film deposition technology. The electrical properties of ZnO thin films depending on the growth conditions for application as an active channel layer of TFT were discussed as a function of ALD process conditions.