A new poly-Si TFT device simulator with quasi three-dimensional (3D) temperatures analysis has been developed. The device characteristic changes caused by the self heating effect in poly-Si TFTs are successfully modeled using this simulator. The influences of various device parameters on the temperature rise caused by the self heating are evaluated in detail. It is found that the substrate materials, the poly-Si thickness and the design rule have significant influences on the temperature rise. It is also confirmed that the temperature rise results in the increased drain current and further increase in the temperature rise gives rise to the reduced drain current the increase of the drain current with the temperature rise in the moderately high drain current region is explained by the reduced barrier height at the grain boundary while the decrease of it in the considerably high drain current regions is done by the decreased electron mobility inside the grain.