The effects of alternate bias stress on p-channel poly-Si TFT`s has been systematically investigated. We alternately applied positive and negative bias stress on p-channel poly-Si TFT`s, device Performance(V$\_$th/, g$\_$m/, leakage current, S-slope) are alternately appeared to be increasing and decreasing. It has been shown that device performance degrade under the negative bias stress while improve under the positive bias stress. This effects have been related to the hot carrier injection into the gate oxide rather than the generation of defect states within the poly-Si/SiO$_2$ interface under alternate bias stress.