Optical imaging of hemoglobin concentration changes in the exposed cortex has been used to investigate the functional brain activation. The concentration changes in oxygenated and deoxygenated hemoglobin can be independently obtained from the dual-or multi-wavelength measurements of the change in reflectance of the exposed cortex and wavelength-dependent optical path length in the cortical tissues. In the previous studies, the partial optical path length were generally estimated by homogeneous and layered models. In reality, the concentration changes in the hemoglobin only occurs in the blood vessels. In this study, the partial optical path lengths in the blood vessels were estimated by the heterogeneous model including the blood vessel structure based upon the image acquired by two-photon microscopy. Light propagation in the exposed-cortex model is simulated to estimate the wavelength dependence of the partial optical path length in the blood vessels. The wavelength dependence of the partial optical path length for the heterogeneous model was different from that for the homogeneous model. In the wavelength range from 500 to 580 nm, the partial optical path length in the blood vessels was mainly affected by the structure of the blood vessels in the region shallower than 50 μm.