A bias voltage stabilizing method for an electricity-optical modulator based on an off-level sampling is disclosed. The method includes the steps of a first step for detecting an electric signal by feed-back an output optical signal of a MZ optical modulator, performing a sample and hold for an OFF level and converting the signal into a digital data, a second step for receiving the sample and hold data with respect to the OFF level, controlling a bias value by performing an up and down count so that a minimum operation level is maintained at near a zero level of an output optical signal based on a binary data and a current data value, and adjusting a bias value, and a third step for outputting a bias control voltage of an optical modulator by converting the bias value controlled by the up and down count into an analog signal, for thereby controlling an adaptive bias voltage irrespective of a traffic characteristic of a signal by selectively extracting an OFF level of an output light, measuring the thusly extracted level and minimizing the value for thereby controlling a bias voltage.