The relationships among morphological parameters of binary polymer blends have been studied. A new equation for correlating morphological parameters : particle size (d), particle size distribution (sigma), particle volume fraction phi, and matrix ligament thickness (T(d,sigma,phi)) is derived. The equation is generally applicable to the binary polymer blends with the log-normal distribution of particle size and the configuration of well-dispersed particles in matrix. The effects of a and particle configuration on T(d,sigma,phi) are discussed. The new equation predicts that T(d,sigma,phi) increases with increasing d and sigma, and with reducing phi. The effect of sigma is found to be influenced by phi. The higher the phi is, the more significant the effect of sigma on T(d,sigma,phi) is. The new equation is applied to the poly(vinyl chloride)/nitrile rubber, polypropylene (PP)/EPDM, and PP/EVA blends. The theoretical predictions agree well with the experimental results. The equations neglecting or underestimating the effect of sigma on T(d,sigma,phi), however, cause great errors. Compared with the effect of sigma, the effect of particle configuration of cubic lattice assumed in deriving the new equation is much less important in the above blends. Therefore, sigma is also an important morphological parameter.