Surface of AZ31B magnesium alloy is melted using a pulsed NdYAG laser in liquid nitrogen. Then the laser melted layer cooled in liquid nitrogen is compared with that cooled in air and the substrate. The results show that the grain size of the laser melted layer cooled in liquid nitrogen is smaller than that cooled in air. The laser melted layer cooled in liquid nitrogen has higher micro-hardness (70～77 HV0.05) than that cooled in air(60～67 HV0.05) compared with the as-received magnesium alloy (about 55 HV0.05). The wear mass loss of the laser melted layer cooled in liquid nitrogen (1×10-3 g) is less than that cooled in air (2×10-3 g). The results indicate that liquid nitrogen is better for the enhancement of wear resistance of the laser melted layer. The experimental results show that the corrosion potential of the laser melted layer cooled in liquid nitrogen and cooled in air decrease by 22 mV and 29 mV than those of the as-received magnesium alloy respectively. This indicates that corrosion resistance of AZ31B magnesium alloys becomes worse by laser surface melting.