Retinopathy of prematurity (ROP) is the major cause of blindness in children. We examined the role of mammalian target of rapamycin (mTOR) on the formation of abnormal retinal blood vessels in a rat model of ROP. To induce ROP model, rats were treated subcutaneously with KRN633, an inhibitor of vascular endothelial growth factor (VEGF) receptor tyrosine kinase, on postnatal day 7 (P7) and P8. The ROP model rats were treated subcutaneously with the mTOR inhibitor rapamycin from P11 to P13. Changes in retinal vasculature, phosphorylated ribosomal protein S6 (pS6), a downstream indicator of mTOR activity, and proliferative status of vascular cells were evaluated on P14 using immunohistochemistry. For comparison, KRN633 was administered according to similar protocols. Rapamycin prevented increases in the arteriolar tortuosity, capillary density, and number of proliferating vascular cells as well as abolished the pS6 immunoreactivity in ROP rats. KRN633 almost completely abolished the abnormalities of retinal vasculature. These results suggest that activation of the mTOR pathway contributes to the onset of ROP-like abnormal retinal blood vessels. Inhibition of mTOR may be a promising approach to treat abnormal retinal blood vessels in ROP.