This paper initially suggests a new capacitance model, i.e., the constant distribution capacitance of photogenerated charge (C.D.C.P.C.). The capacitance originating from the distribution of photogenerated charge in a junction region does not connect with alternative electric signals applied on the junction. It essentially distinguishes from the depletion and diffusion differential capacitances as previously known. Since the photo-generated charge, Q, and the photogenerated voltage, V, are connected by the equation, Q-CV, where C is just the C.D.C.P.C. the importance of this new capacitance model is self-evident. In the paper, the expressions of C.D.C.P.C. under the condition of approximately abrupt and graded junctions are deduced respectively at low forward bias. As the examination of the validity of the C.D.C.P.C. model, the deduced results are applied to the contactless test method of the device leakage and a good agreement between the theoretic calculation and experimental results is obtained, which enables the quantitative determination of the device leakage by the high frequency photoconductor decay method to be possible, The paper initially suggests a new capacitance model, ie, the constant distribution capacitance of photogenerated charge (CDCPC). The capacitance originating from the distribution of photogenerated charge in a junction region does not connect with alternative electric signals applied on the junction. from the depletion and diffusion differential capacitances as previously known. Since the photo-generated charge, Q, and the photogenerated voltage, V, are connected by the equation, Q-CV, where C is just the CDCPC the importance of this new capacitance model is the self-evident. In the paper, the expressions of CDCPC under the condition of approximately abrupt and graded junctions are deduced respectively at low forward bias. As the examination of the validity of the CDCPC model, the deduced results are applied to the contactless test method of the device leakage and a good agreement between the theoretic calculation and experimental results is obtained, which enables the quantitative determination of the device leakage by the high frequency photoconductor decay method to be possible,