The goal of this study was to investigate the effect of photothermal laser irradiation on rat breast tumor (DMBA-4) vascular contents. An 805-nm diode laser was used in our experiment with a power density ranging from 0.32 to 1.27 W/cm². The dynamic changes of oxygenated hemoglobin and total hemoglobin concentrations, Δ[HbO₂] and Δ[Hb]_total, in rat tumors during photothermal irradiation were noninvasively monitored by a near-infrared spectroscopy system. A multichannel thermal detection system was also used simultaneously to record temperatures at different locations within the tumors. Our experimental results showed that: (1) photoirradiation did have the ability to induce hyperthermic effects inside the rat breast tumors in a single exponential trend; (2) the significant changes (P < 0.005) of Δ[HbO₂] and Δ[Hb]_total in response to a low dosage of laser irradiation (0.32 W/cm²) have a single exponential increasing trend, similar to that seen in the tumor interior temperature; and (3) the increase in magnitude of Δ[HbO₂] is nearly two times greater than that of Δ[Hb]_total, suggesting that photoirradiation may enhance tumor vascular oxygenation. The last observation may be important to reveal the hidden mechanism of photoirradiation on tumors, leading to improvement of tumor treatment efficiency.