This paper investigates the harmonic interactions of current controllers in multi-paralleled grid-connected inverters. Potential harmonic instability phenomenon, which features oscillations above the fundamental frequency are evaluated by the impedance-based stability criterion. The possible reasons of the destabilized inverter with different types of grid impedances are explained by the passivity based analysis and impedance-based stability criterion. Thereafter, case studies are provided which shows how the non-passive grid connected inverters interact with the varying grid impedance due to load changes or by the new connections of other grid-connected inverters. Time domain simulations in the PSCAD/EMTDC environment and experimental results shows the corresponding harmonic interaction problems may exist in nowadays power system and the impedance-based stability analysis can be an effective tool to assess those problems.