Very high cycle fatigue (VHCF) properties of high-pressure die cast Magnesium alloy AZ91HP have been investigated. Ultrasonic fatigue tests up to 109 cycles were conducted at the loading frequency of 20 kHz, under R=-1 condition and in ambient air. The experimental results show that specimens fail even after 107 cycles although the scatter seems to be large probably due to the presence of materials defects. However, there seems to be a fatigue limit at about 109 cycles. The fractures contain typical brittle features, with the fatigue cracks seen to initiate from the porosity in the material, either from the surface or beneath.