The nature of performance instabilities for the pentacene-based organic thin film transistor (OTFT) was researched in this study. It is shown the extraction of the threshold voltage (VTH) and field-effect mobility (μ) from an analysis of the time-dependent output (transfer) characteristics. It is found that the drain-source voltage variation may lead to a change in μ, owing to hole trapping in the pentacene film. The gate-source voltage (VGS) variation may lead to a shift of VTH, owing to electron trapping at the pentacene/dielectric interface. We found the discrepancy in the time-dependent output and time-dependent transfer characteristics, implying that a change in μ (a shift of VTH) is bulk (interface) related rather than interface (bulk) related.