In this study, partially debonded spherical particles in a particulate composite are analyzed by three-dimensional finite element method to investigate the effect of debondings on the composite stiffness, and the way to replace a debonded particle with an equivalent inclusion is examined. The variation in Young's modulus and Poisson's ratio of a composite with the increase of the debonded angle was evaluated for different particle arrangements and particle volume fractions, which in turn compared with the results derived from the equivalent inclusion method. Consequently, it was found that by replacing a debonded particle with an equivalent othotropic one, the macroscopic behavior of the damaged composite could be reproduced so long as the interaction between neighboring particles is negligible.