Abstract: Objective To observe the radiosensitizing effect of recombinant human endostatin （endostar） on rhabdomyosarcoma xenograft， and to further explore the underlying mechanism. Methods The subcutaneous xenograft model of femal BALB／C mice with A673 rhabdomyosarcoma was successfully established， and the mice were randomly assigned to four groups： the control group， the endostar group， the radiotherapy group， and the endostar combined with radiotherapy group. The condition of the xenografts were observed and measured to evaluate the antitumor effect. Immunohistochemical assay was used to determine the expressions of vascular endothelial growth factor （VEGF） and microvessel density （MVD） in each group. Results The tumor inhibition rates of the endostar group， the radiotherapy group and the endostar combined with radiotherapy group were 13.16％， 23.82％ and 34.04％， respectively. The radiotherapy group and the endostar group had the inhibitory tendency， but the difference was not significant compared with the control group（P＞0.05）. Significant antitumor activity was found in the endostar combined with radiotherapy group compared with the control group（P＜0.05）. The MVD of the control group， the endostar group， the radiotherapy group and the endostar combined with radiotherapy group were 21.16±6.53, 13.02±1.99, 12.21±1.58, 7.84±0.91， respectively. The MVD expression were reduced remarkably in all the treatment groups compared with the control group（P＜0.05）， with the endostar combined with radiotherapy group the most significant（P＜0.05）. And the VEGF levels of the control group， the endostar group， the radiotherapy group and the endostar combined with radiotherapy group were 2.01±016，1.46±0.46，3.37±0.74，2.41±0.21，respectively. The VEGF level in the radiotherapy group was higher than those in the control group and endostar group. The level of VEGF in the endostar combined with radiotherapy group decreased remarkably compared with the radiotherapy group（P＜0.05）. Conclusion Endostar in conjunction with radiotherapy can significantly restrict the growth of rhabdomyosarcoma xenograft， and the underlying mechanism may include the inhibition of angiogenesis， the decrease of MVD and the expression of VEGF induced by radiation.