Tumor vasculature creates a hostile tumor microenvironment (TME) in vivo and nourishes cancers, resulting in cancer progression and drug resistance. To mimic the biochemical and biomechanical environments of tumors in vitro, several models integrated with a vascular network have been reported. However, the tumor responses to biochemical and biomechanical stimuli were evaluated under static conditions and failed to incorporate the effects of blood flow to tumors. In this study, we present a tumor-on-a-chip platform that enables the evaluation of tumor activities with intraluminal flow in an engineered tumor vascular network. The fibroblasts in the tumor spheroid induced angiogenic sprouts, which constructed a perfusable vascular network in a tumor spheroid. The perfusability of the engineered vascular network was preserved during the culture. Moreover, perfusion for over 24 h significantly increased the proliferation activities of tumor cells and decreased cell death in the spheroid. Drug administration under perfusion condition did not show the dose-dependent effects of anticancer drugs on tumor activities in contrast to the results under static conditions. Our results demonstrate the importance of flow in a vascular network for the evaluation of tumor activities in a drug screening platform.