A method based on Kelvin probe force microscopy for measuring minority-carrier diffusion length in semiconductors is described. The method is based on measuring the surface photovoltage between the tip of an atomic force microscope and the surface of an illuminated semiconductor junction. The photogenerated carriers diffuse to the junction and change the contact potential difference between the tip and the sample, as a function of the distance from the junction. The diffusion length L is then obtained by fitting the measured contact potential difference using the minority-carrier continuity equation. The method was applied to measurements of electron diffusion length in GaP $\mathrm{pn}$ and Schottky junctions. The measured diffusion length was found to be $\ensuremath{\sim}2 \ensuremath{\mu}\mathrm{m},$ in good agreement with electron beam induced current measurements.