This paper, focus on the study and design of a shunt active power (APF) filter for the compensation of harmonic currents and reactive power in polluted environment and under unbalanced network voltage. In APF design and control, p-q theory was often served as the basis for the calculation of compensation current. In this theory, the mains voltage was assumed to be an ideal source in the calculation process. However, in most of time and most of industry power systems, mains voltage may be unbalanced and/or distorted. For these reasons, we used a new technique with instantaneous power theory (p-q theory) based on Phase Locked Loop (PLL) in order to control APF under unbalanced mains voltage. A pulse width modulation (PWM) technique is used to generate the switching signals to the shunt APF.-This paper, focus on the study and design of a shunt active power (APF) filter for the compensation of harmonic currents and reactive power in polluted environment and under unbalanced network voltage. In APF design and control, p-q theory was often served as the basis for the calculation of compensation current. In this theory, the mains voltage was assumed to be an ideal source in the calculation process. However, in most of time and most of industry power systems, mains voltage may be unbalanced and/or distorted.　For these reasons, we used a new technique with instantaneous power theory (p-q theory) based on Phase Locked Loop (PLL) in order to control APF under unbalanced mains voltage. A pulse width modulation (PWM) technique is used to generate the switching signals to the shunt APF.