MgAl2O4 layer with several decade nano-meter in thickness was coated uniformly on Al18B4O33 whisker by vapor-deposition of pure magnesium in vacuum and then heat-treated in atmosphere in order to prevent to the direct reaction between Al18B4O33 whisker and molten ZK60 (Mg-(4.8-6.2)%Zn-(0.45-0.8)%Zr) alloy in preparing the composites. By the coating, the composite have no interfacial reaction in whisker and matrix. As the consumption of Zn at the interface was not observed during preparation of the composites, the composites have good aging behavior, but showed the shortening of the time to reach the peak hardness. As the cooling speed of the composite after solution treatment is slow because of low thermal conductivity of the composites, some precipitation (MgZn2) was observed in the composites. Furthermore, the precipitation grew around the interface between whisker and matrix preferentially because of thermal stress around the interface. It seems these phenomena affect to the shortening of the time to reach the peak hardness.