Objective To access the effects of post′s diameter on the retention of post-core restorations.Methods Thirty- three simulant rootswith same length,taper and different diameter (1·305 mm, 1·830 mm, 2·175 mm and 2·700 mm respective- ly) were made of polymethylmethacrylate and were divided into 4 groups. The wax patterns of post-cores were manufactured and cast. The post-cores were cemented. After that, the adhesion, frictional force and restriction of the post-core restorations were measured.Results When the proportion of length and diameterwas smaller than 4·372, the linear regression between the post′s diameter and the retention of post-core crown systemwas F=107·98+72·08D. The linear regression between the post′s diameter and the adhesion was P=9·60+2·22D. The linear regression between the post′s diameter and the frictional force and restriction was Q=98·37+69·87D. The statistical analysis of the results indicated significant difference (P
