An operation voltage auto-adjustable circuit for active matrix organic light emitting diode ("AMOLED") and an auto-adjusting method thereof is provided. The circuit for automatically adjusting a operation voltage of an AMOLED includes a display panel of an AMOLED having a terminal of an organic light emitting diode ("OLED"); and an auto-adjusting circuit connected to the terminal of the OLED, wherein a current passing through the terminal of the OLED is detected by the auto-adjusting circuit, and a voltage applied to the terminal of the OLED is adjusted by the auto-adjusting circuit according to the current detected. Therefore the current passing through the terminal of the OLED is sensed to automatically adjust the voltage applied thereto, and the current passing through the terminal of the OLED is stably maintained.