Here, we propose a simple method for measuring the built-in potential and its temperature dependence of a photodiode by a photosaturation technique. The experimental design facilitates both current-voltage and null voltage measurements as a function of white light intensity. This method gives the built-in potential directly; as a result its temperature dependence can conveniently be measured by using commercial photodiodes. The values obtained are quite close to the expected ones. Therefore, the technique can easily be adopted as a laboratory experiment in graduate/undergraduate courses to measure the built-in potential. (Contains 5 figures.)