It has been reported that long circulating liposomes enhanced the antitumor effect of doxorubicin (DOX) by increasing delivery of DOX to tumor tissues. However, there is no quantitative information on the relationship between the antitumor effect and liposomal characteristics governing the release rate of entrapped drugs, although the importance of drug release-rate control from liposomes has been pointed out. Here, we developed a physiological model for free and liposomal DOX to calculate the time course of free DOX in the extracellular space and linked this with a cell kill kinetic model to quantify the antitumor effect of DOX. Simulations were performed to clarify the relationship between antitumor effect and pharmacokinetic or physicochemical parameters of liposomes, as well as pharmacological or physiological parameters of tumor tissues. The importance of long circulation time of liposomes was confirmed. The optimum rate of drug release from long circulating liposomes was found at the release rate constant of around 0.06 h(-1). This optimum value was not dependent on the tumor proliferation time, sensitivity of tumor cells to DOX, or the tumor blood flow-rate. This simulation indicated that the optimization of the delivery to tumor tissue by long circulating liposomes could be possible by changing the release rate of DOX for the maximum antitumor effect.