IVIVC (In vitro/in vivo correlation) is useful for predicting in vivo results from in vitro data. The aim of this study was to develop IVIVC of sustained release diltiazem. For this purpose, three types of diltiazem tablets with different in vitro dissolution rates were prepared. An in vitro dissolution testing method comprising of paddle apparatus, 50 rpm, water as dissolution medium was developed. Under these condition, we demonstrated that AUCinf could be predicted by evaluating $d_{70%}$ (time dissolved 70%) in vitro since the in vivo AUCinf was correlated with the in vitro $d_{70%}$ (r=-0.9981).