The aim of this study was to investigate the relationship between blood flow and skin temperature after water heating. Simultaneous measurements of skin temperature, blood pressure, and blood flow in hands and forearms exposed to the air were carried out after the test hands were immersed into the water at 40℃ for 2〜5 min. Temperatures were measured by an infrared thermography. Blood flow in the ulnar artery and blood perfusion in the fingertip were monitored by an ultrasound Doppler and laser Doppler velocimeter. Results indicate that water heating can induce the arterial blood flow in the forearm to either increase or decrease; when the thermal stress is raised to be above a critical value for different subjects, the arterial blood flow would be decreased. Additionally, the spectrum analyses of the sequential thermal images show that with the thermal stress increasing, the amplitude of some frequencies become stronger, which implies that the hemodynamic effect on the skin temperature tends to be stronger.