Threshold voltage shift caused by trapping and release of charge carriers in a thin-film transistor (TF1`) is implemented in AIM-SPICE tool. Turning on and off voltages are alternatively applied to a TFT to extract charge trapping and releasing process. Each process is divided into sequentially ordered processes, which are numerically modeled and implemented in a computer language. The results show a good agreement with the experimental data, which are modeled. Since the proposed method is independent of TFT`s behavior models implemented in SPICE tools, it can be easily added to them.