Blood now dynamics in microvessels in a small animal tumor model provide important physiological and pathological information. In this study, red blood cell velocity in microvessels distributed near the surface in a rodent tumor model is investigated using an approach based on optical flow estimation. A long-term experiment is performed to observe tumor growth on nude mice and to monitor the variation of blood flow velocity. The blood now images are acquired using a capillaroscope with aspatial resolution of 1.42 um and an image sampling rate of 30 frames per second. Frame to frame image registration with mutual information feature matching is used to stabilize images to eliminate the vessel shill that result s from regular heartbeats, The blood !low results measured by the optical !low method are compared with visual inspection results to verify ac curacy. The blood flow observations focus on the tumor surface of nude mice. Multiple weekly data are acquired from each nude mouse for five weeks after tumor implantation. The average red blood cell velocities of capillaries are measured and calculated at different weeks for each tumor. The distribution of blood flow velocity among all samples ranges from 40 to 350 um/s. The differences in vessel diameter and blood flow velocity du ring tumor growth are studied. A significant difference is found between the velocity values at the initial week and the 2nd week. The results suggest that an optical-based method can be used to develop experimental protocols for the numerical simulation of tumor growth. The proposed measurement system may also be useful in many other biotech no logical evaluations.