Abstract: Using the photovoltaic spectral response of epitaxial P-N junction, the paper suggests a method of determining the minority carrier diffusion length in N layer of N/P epitaxial silicon wafer. The authors made a group of diagrams which contained the information of minority carrier diffusion lengths in the epitaxial layer and the substrate according to the theoretical analysis. When there is a thermal SiO₂ thin film on the surface of the sample, the photovoltage of the sample depends on the characteristics of the epitaxial P-N junction.