Objective To compare the effects of flap delay and vascular endothelial growth factor (VEGF) on the viability of the rat dorsal flap. Methods Thirty rats were divided into 3 groups: saline group, flap delay group and VEGF group. The rats in flap delay group underwent flap delay by keeping bipedicle untouched, and the cranial pedicle was cut 7 days later. The rats in VEGF group were given VEGF solution locally when the flaps were elevated in the operation. The ratsin saline group were given saline solution in the same way. Five days after thesingle pedicle flaps were performed, the flap survival rate was measured. Theflap tissues were collected to measure and analyze the microvascular density, diameter and sectional area by immunochemical method. Results The flap survival rate of flap delay group was similar to that of VEGF group andthere is no statistically significant difference(Pgt;0.05). The vascular diameter of flap delay group was much larger than that of saline group and VEGF group, showing statistically significant difference (Plt;0.05). The vascular density of VEGF group was much higher than that of saline group and flap delay group, showing statistically significant difference (Plt;0.05). The vascular sectional area of flap delay group was similar to that of VEGF group(Pgt;0.05). Conclusion The change in the flap after flap delayis manifested as obvious dilatation of microvessels, while the change in the flap after the injection of VEGF is manifested as obvious vascular proliferation. Both flap delay and VEGF can increase the vascular sectional area and the viability of the flap, but the mechanism is different.