This work concerns the electrical properties and carrier transport behavior of polycrystalline pentacene-based thin film transistors (TFTs) in air and high-vacuum environments. The transistor in a high vacuum outperforms that in air. The dependence of the field-effect mobility on the gate voltage and the drain voltage is considered. A potential barrier model is applied to estimate the trap density of the pentacene transistor. The determined trap density at the grain boundaries of the pentacene film in air exceeds that in a high vacuum. These results show that the increased trap concentration at the grain boundaries in ambient air limits carrier transport.