Aluminum borate whisker (9Al2O3 · 2B 2O3: AlBO) is relatively low cost and has high strength. Carbon fiber (CF) has high thermal conductivity and high temperature strength. Therefore, CF and AlBO hybrid reinforced magnesium alloy composites would be lightweight and expected to have superior high strength and high thermal conductivity. It is known that wettability between ceramic fiber and metal is important factor to fabricate metal matrix composites. Previously, poor wettability of graphite/magnesium system and good wettability of graphite/magnesium system have been reported. However, the initial contact angle between CF+AlBO hybrid preform and molten magnesium was not obvious. Moreover, whether magnesium infiltrate into hybrid preform spontaneously or not has not been clarified. In this study, criteria of infiltration of molten magnesium into the hybrid preform were examined. As a result, the initial contact angle between the hybrid preform and molten magnesium followed with Cassie's rule of mixture approximately. Molten magnesium infiltrated into CF+AlBO hybrid preform spontaneously. Moreover, all the estimated threshold pressure to infiltrate into the preform was negative. This conformed to experimental results.