We report on the electrical properties of bottom-gate InGaZnO (IGZO) thin film transistors (TFTs) with different channel layer thicknesses. The IGZO channel layer with thickness varied from 25 to 120nm were deposited by radio frequency sputtering. Al 2 O 3 films were deposited on highly-doped n-Si substrate by atomic layer deposition (ALD) as gate insulator in this work. The influence of the IGZO channel layer thickness on the performance of TFTs is studied. The performance of devices is found to be thickness dependent. The best performance of devices is obtained from a 58nm thick IGZO TFT, which shows a field-effect mobility in the saturation region of 6.2cm 2 /Vs, a threshold voltage of 2.1V, an I on /I off ratio of approximately 6.4×10 7 , and a subthreshold swing of 0.6V/dec.