In this study, the change of tumors' chemical composition in the temperature range of 20 ∼70°C is quantified for photothermal tumor therapy by photoacoustic spectroscopy (PAS) with the wavelengths of 700∼1000 nm. Based on the measured photoacoustic signals, two absorption peaks at the wavelengths of 750nm and 950nm are identified. It is also observed that high temperature (>55°C) is able to induce the physical and chemical degeneration of tumors. According to the in vitro tests, a new chemical species, met-hemoglobin, which is absent in normal blood, is generated at high temperature with enhanced near-infrared absorption.