In this article, we investigate hysteresis behavior observed in the current-voltage transfer curve of regioregular poly(3-hexylthiophene)(P3HT) based thin film transistors (PTFTs) using bi-directional voltage sweeping. According to the results of our experiments, the hysteresis effects were closely related to oxygen (O2) and water (H2O) molecules residing in the P3HT active layer. As a result, we hypothesized that O2 and H2O in the P3HT could form two kinds of dopants, i.e., stable and transient states. Stable state dopants, which are commonly thought to be of the acceptor-type dopants, could induce holes in the channel region contributing to better on-current. On the contrary, transient state dopants were found to be unable to induce holes, however could enable the formation of polarization agents and create hysteresis. These two types of dopants seem have a common origin, thus they could be converted into each other through the combination of specific processes and applied external bias. From the viewpoint of device operation stability, except on-current level and characteristics including threshold voltage, off-current magnitude and onoff ratio, the impact of hysteresis should garner the attention of those pursuing high performance PTFTs.