Abstract: The effect of the content of nano particle and micro particle on mechanical properties of various level dispersing phase reinforced ceramic composites was investigated. The result has revealed that the improvement of strength and toughness is related with the content of two kinds of particles and their relative ratio. The length of critical crack of multiphase ceramics is also characterized by optimal analyzed method based on the theory of fracture mechanism derived by Griffith. The predicted microcrack length becomes short with the content of nano particles ranging from 3vol% to 12vol%. Furthermore, it is enlarged with increase of the content of micro particles. The phenomenon of different optimal volume fraction of particles between the composition of maximum strength (7.9vol%, 7.9vol%) and that of toughness(9.7vol%, 9.7vol%) may be attributed to the stronger sensibility of strength to internal defects than that of toughness to these defects formed in the process of preparation.