The conventional passive filtering has been outdated since it detune with age, this may affect its filtering performance and also cause resonance problems, active power filters (APFs) are attractive solutions to solve harmonic-related power quality problems due to the development of power semiconductors, control theory and digital signal processors (DSP), and they have become the first choice for power quality solutions. This paper systematically investigates the 3-phase 3-wire active filter in 3-phase 3-wire and 3-phase 4-wire systems, and both the balanced and unbalanced situations are considered. It has been found by various experiments, that 3-phase 3-wire active filter works quite well in 3-phase system, however, the performance of 3-phase 3-wire active filter in 3-phase 4-wire system is unsatisfactory, and it may even inject unwanted harmonics to the grid under certain situations, which deteriorates power quality of distribution system.