Abstract: In order to obtain better mechanical properties, fine particles in several micron or submicron diameter were used as reinforcement to reduce stress concentration usually induced by larger particles. Zn-22%Al matrix composites with alumina particulate reinforcement were fabricated by high intensity ultrasonic treatment. The diameters of particulates were different and the minimum one was 0.5 micron. Comparing the height changes of composites before and after remelt which have been degassed with semisolid compression, the relation between apparent viscosity and reinforcement diameter was obtained. Experiment results show that there is no interface reaction product involved in the apparent viscosity change. The smaller the particulate, the bigger the viscous force. That is the dominating reason for the composite melt apparent viscosity increment. And the gas introduced with particulate is another reason.