Abstract: The bulk Mg-alloy, AZ91 D, was modified by adding the Cu-coated SiC powder in a pre-drilled hole and by passing a strong pulsed current at a high temperature. The effect of the pulse current on the microstructures, phase-structures and micro-hardness was investigated with X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy and hardness indentation. The results show that the pulse current had a major impact. For example, the modified AZ91 D alloy comprised α-Mg matrix, divorced eutectic β-Mg₁₇Al₁₂ at grain boundaries, reinforced phase Mg₂Si along β-phase and a little Cu around the β-phase. As the peak current increased, the grain-size changed in a decrease-increase mode, decreasing to the smallest at 1500 A and increasing in 1500~2000 A range.In contrast, the micro-hardness varied in an increase-decrease manner. A peak current of 1500 A increased the micro-hardness from an average of HV_4.959. 73 of the control sample to the highest of HV_4.971. 08.