Green laser pulses at a wavelength of 532 nm from a Q-switched Nd:YAG laser were employed as irradiation sources for photoacoustic tomography (PAT). The vascular structure of the brain was imaged clearly, with optimal contrast, because blood has strong absorption near this wavelength. The photoacoustic images of rat brain tumors in this study clearly reveal the angiogenesis that is associated with tumors. Brain tumors can be identified based on the distorted vascular architecture of brain tumorigenesis and related vascular changes, such as hemorrhage. This research demonstrates that PAT can potentially provide a powerful tool for small-animal biological research.