Thin film transistors with different channel thickness were fabricated with metal induced lateral crystallization (MILC) and conventional solid-phase crystallization (SPC). The performance of MILC devices is much better than that of SPC devices, and the performance of the MILC devices can be further improved by reducing the thickness of the channel layer. Leakage current less than 10-13 A/μm and on/off ratio above 3×107 are obtained for MILC device with ultra-thin channel layer. Field effect mobility is improved from 80 cm2/Vs for NMOS or 51 cm2/Vs for PMOS to 110 cm2/Vs by using ultrathin channel layer. Possible reasons are proposed and discussed.