The standard diffusion length （Lp） test procedure is apt to damage the p-n junction, so we often use characteristic decay length （L） obtained by laser beam induced current　（LBIC） instead of Lp. Two dimensional modeling is used to get the relations of L and Lp. Calculated L/Lp ratio is about 1.1, and it is not affected by doping concentration, Shockley-Read-Hall （SRH） lifetime and mobility. For HgCdTe photodiodes, we get characteristic decay length from the LBIC experiment, and get the true diffusion length by division by the ratio 1.1.