An apparatus and method thereof for measuring an output current from a primary side of a power converter are provided. A peak detector is designed to sample a peak value of a converted voltage of a primary-side switching current. A zero-current detector detects a discharge-time of a secondary-side switching current through an auxiliary winding of a transformer. An oscillator generates a switching signal for switching the power converter. An integrator generates an integrated signal by integrating the converted voltage of the primary-side switching peak current with the discharge-time. The time constant of the integrator is correlated with the switching period of the switching signal. The integrated signal is thus proportional to the output current of the power converter.