To analyze the hysteresis phenomenon in p-channel low-temperature polycrystalline-silicon thin-film transistors (LTPS TFTs), the direct correlation between the hysteresis and the interface (Nit) and the grain-boundary trap density (Ntrap) has been investigated. To fabricate LTPS TFTs with different electrical properties and trap types, the thickness of a-Si was varied from 30 to 80 nm and crystallized by the excimer-laser-anneal (ELA) method. The interface trap density is extracted from the subthreshold slope (SS) and low-high-frequency C-V analysis, while the grain-boundary trap density is extracted by the Levinson and Proano method. The LTPS TFTs with smaller hysteresis exhibited a lower trap density. From the correlation between extracted parameters, the hysteresis seems to be more dependent on Nit and decreases when the film thickness increases to 80 nm while the Ntrap is almost the same in all devices.