Abstract: Effects of Ca addition on microstructure and mechanical properties at middle temperatures (150-250 ℃) of heat-resistant magnesium alloy AE42 were investigated by optical microscopy, scanning electron microscopy and energy dispersive spectroscopy and tensile testing. The results indicate that the second phase in AE42 alloy was mainly composed of acicular Al11Ce3 and short rod shaped Al4Ce. With the increase of calcium content, the amount of acicular Al11Ce3 decreased, a new phase of fishbone shaped Al2Ca appeared and increased. Moreover, the mechanical properties of test alloys at middle temperatures got better when calcium content rose. And the alloies with 1.2% Ca content had the best comprehensive mechanical properties.