We demonstrated a bias stress effect on organic thin-film transistors (OTFTs) that causes device degradation, including a large shift in the threshold voltage and hysteresis in the transfer characteristics. Specifically, we analyzed the electrical characteristic variations in pentacene TFTs with a poly(methyl methacrylate) gate insulator under different bias stress conditions. We found that bias stress between the gate electrode and source/drain electrodes affects the charge transport properties, whereas bias stress between the source and drain electrodes influences the charge injection properties.