an led illuminant driving circuit and an automatic brightness compensation method thereof are provided herein . the automatic brightness compensation method includes : providing a target value ; detecting an operation period of a pulse of an output of the led illuminant driving circuit , the pulse is adapted to an led illuminant for making the light emitting ; deciding a peak value according to the target value and the operation period ; and setting a peak level of the pulse according to the peak value . the led illuminant driving circuit and the automatic brightness compensation method thereof provides a stable average current / voltage to the led illuminant and avoids brightness variations of the light emitting .