In this study, we monitor the progress of vasculature in early tumor growth using photoacoustic imaging over a 20 day period after subcutaneous inoculation of breast cancer tumor cells in a mouse. With 532 nm laser pulses employed as an irradiation source, the photoacoustic images were obtained through the photoacoustic signals received by a hydrophone in orthogonal mode. The morphological characteristics of vasculature in tumor region are clearly resolved in the photoacoustic images, and the change in structure as well as the increase in density can be identified. Moreover, the average photoacoustic signal strength of vasculature in tumor region, which is highly correlated with the total hemoglobin concentration of blood, is enhanced during early tumor growth. These results indicate the feasibility of detecting early stage tumor and monitoring the progress of anti-angiogenic therapy by photoacoustic imaging.