To investigate the degree of the effect of the interfacial thermal resistance between the matrix and the reinforcement, the effective thermal conductivity of aluminum matrix composites (Al/SiC, Al/TiB₂, Al/Al₂O₃ and Al/SiO₂) was calculated with the new simulation code which can take account of heat transfer at the interface. The critical element size was defined by a simple equation, L_cr = h/λ, where λ is the harmonic mean of thermal conductivities of the reinforcement and the matrix, and h is the coefficient of heat transfer between the reinforcement and the matrix. This critical element size is important value to design composites. If the size of the reinforcement is smaller than the critical element size, it is predicted that the effective thermal conductivity will decrease by the interfacial thermal resistance. On the other hand, if the size of the reinforcement is large enough, the effective thermal conductivity will not decrease. The validity of the critical element size for aluminum matrix composites was confirmed in this study.