The lifetime-based method has been applied to Pressure-Sensitive Paint (PSP) measurement in low-speed flow. Based on theoretical considerations and sample tests, a method to calculate pressure using "a-priori" method was proposed. In this method, pressure is related to the ratio of the luminescent intensity ratios obtained at two different gates between the windon and wind-off conditions, multiplied by a temperature correction factor calculated from the single-point measurement of model substrate temperature. The sample tests showed that the effect of temperature as well as non-uniformity of lifetime could be cancelled out by the proposed method. In order to evaluate the effectiveness of this method, a simplified car model was tested in a low-speed wind tunnel. The result obtained by the lifetime method was in good agreement with those of pressure taps and the intensity method.