Objective: To explore the in vitro antitumor effect of cisplatin-loaded micelles on osteosarcoma cell line U-20 s. Methods: Cisplatin-loaded micelles were prepared by a nano-precipitation method with poly (ethylene glycol)-polycaprolactone (mPEG-PCL) as drug carrier. In vitro cytotoxicity of cisplatin-loaded micelles against osteosarcoma cell line U-20s was assessed by MTT assay. Results: The size of cisplatin-loaded micelles is less than 100 nm. The drug loading content is nearly 5% with the encapsulation efficiency more than 18%. In vitro stability test shows cisplatin-londed micelles are stable at routime time. In vitro release study indicates that cisplatin can be released from the core-shell structure of polymeric micelles in a sustained manner. In vitro cytotoxicity assessment shows that free cisplatin and cisplatin-loaded micelles display similar time and dose dependent cytotoxicity against osteosarcoma cell line U-20s. The IC50 value of cisplatin-loaded micelles is bigger than that of free cisplatin at each corresponding time. Conclusion: In vitro evaluation of cisplatin-loaded micelles demonstrates effective antitumor effects compared with free drugs. Therefore,development of nanosized drug delivery systems emerges as a novel field in the research of anticancer drugs,which merits more intensive studies and reveals potential application.