Inhibiting or circumventing drug resistance by using drug delivery systems (DDSs) such as micelles has attracted significant attention recently. In this present study, a polyvinyl caprolactam-polyvinyl acetate-polyethylene (Soluplus(®)) micelle was developed as the delivery system for doxorubicin (DOX) and evaluated both in vitro and in vivo. In vitro, Soluplus(®) micelles could significantly enhance the cellular accumulation of DOX in MCF-7/DOX cells, meanwhile, P-glycoprotein (P-gp)-mediated drug efflux was inhibited which was also verified in the membrane fluidity study. And MCF-7/DOX cells were found to be more susceptible to the cytotoxic effects of DOX-M. In vivo, both the P-gp inhibitors verapamil and Soluplus(®) could improve the cytotoxicity of DOX·HCl in MCF-7/DOX tumor-bearing mice, which were further certified by the effect of Soluplus(®) on P-gp inhibition. Furthermore, the excellent antitumor efficacy of DOX-M by intravenous injection was also observed, which indicated that the P-gp inhibition effect of Soluplus(®) could enhance the susceptibility of resistant tumor to DOX in vivo. In conclusion, our study suggested that Soluplus(®) micelles might be an applicable drug delivery system for enhancing the antitumor efficacy of P-gp substrates.