Objective To investigate the effect of vascular endothelial growth factor (VEGF) expression level and sources on tumor-induced angiogenesis by numerical simulation. Methods A two-dimensional discrete mathematical model of tumor-induced angiogenesis was developed to simulate the growth of microvascular networks inside and outside of the tumor, focusing on endothelial cell proliferation, degradation, random motility, chemotaxis, haptotaxis and stromal-derived VEGF and tumor-derived VEGF. The relationship between VEGF derived by each compartment and tumor microvessel density was discussed. Results The high VEGF region was consistent with proliferating cell and tumor periphery regions in which microvascular density was also high. The simulation demonstrated that an enlargement of proliferating cell region could lead to higher VEGF expression level and higher microvascular density. However, for different types of tumors, the correlations between different VEGF expression sources and microcascular density were not significant. Conclusions The effect of VEGF expression level and source on VEGF-mediated angiogenesis can be investigated by the proposed model. Particularly, taking VEGF expression from different sources into consideration could be a useful modeling tool for anti-VEGF targeted therapies.