Material characteristics of powder compacts are closely related to the packing structure of particle aggregates. In order to express the microscopic structure, a probability function of regular-shaped voids in the interspaces among particles over a cross section of the compact was studied theoretically and experimentally. It was found that the regular shape of the voids is normally circular, and that the void diameter becomes small and uniform for low voidage and narrow size distribution of particles. The reduction and uniformity of void diameter has been attributed to an increase in the number of smaller particle sections. The actual existence probability of any void is lower than the theoretical prediction because of the ineffective void spaces which are formed in the packing of regular-shaped voids among interparticle void spaces. The difference between theoretical and experimental probabilities of voids was correlated by a simple equation based on the total void spaces available.