By investigating the variation of different characteristic parameters of pentacene/poly(methyl methacrylate)transistors suffered from electric stress in an environment without O2, H2O and light, we deduce lifetimes of the transistors by different criterion parameters. Defined by the time for the parameters changing one half,the lifetime is different from the minimum of 7h (using on/off current ratio as the criterion parameter) to the maximum of 2.38 × 108h (using transconductance as the criterion parameter). We also find that, under our experimental conditions, the main reason that affects the stabilities of the device is the increase of shallow traps formed in the organic semiconductors.