According to the microstructures of small grain shellproof ceramic composite, the effective self-consistent method is used to determine the effective stress field of the shellproof ceramic composite. Based on the effective stress field and fracture feature of small grain shellproof ceramic composite, considering the mechanism of growth and coalescence of microvoids, crack nucleation model of the shellproof ceramic composite is built. Under the effect of shearing stress, dislocations slip along slip plane and pile up in matrix grain boundary. Owing to the stress concentration before leading dislocation, there is a stress field around the stress concentration. Letting the maximum tensile stress equal to the theoretic fracture strength of composite matrix, the ultimate stress of small grain shellproof ceramic composite is obtained.