This study presents the air influenced electrical properties of poly (3-hexylthiophene) (P3HT) film in a transistor structure. In order to observe the effect of air exposure on P3HT films, the current-voltage characteristics of P3HT-based organic thin-film transistors (OTFTs) were obtained by increasing the air-exposure time. Upon increasing the air exposure time up to 92h, the drain current in the saturation region increased, and a positive shift of the threshold voltage was observed. On the other hand, a decrease in the drain current and a negative shift of the threshold voltage were observed after 92h. A simple theoretical approximation based on carrier doping and scattering effect describes the relationship between electrical performance of P3HT films and air exposure time.