A method for measuring a slope angle of a vehicle includes, by a controller: determining whether or not a speed of the vehicle is 0 based on a speed signal received from a vehicle speed sensor of the vehicle; measuring an extremal value measured first, an extremal value measured second, and an extremal value measured third among extremal values of an output signal of an acceleration sensor detecting a signal corresponding to the slope angle of the vehicle when the speed of the vehicle is 0; verifying whether or not the measured three extremal values satisfy conditions of the extremal values based on reference conditions of each of the extremal values; estimating a steady state value of the output signal of the acceleration sensor, based on the extremal values measured first and second among the verified three extremal values and a dynamic characteristic parameter of the vehicle or based on the verified three extremal values; and converting the steady state value into the slope angle of the vehicle. The output signal of the acceleration sensor has a damped free vibration waveform.