The dark current mechanism of B + implanted n on p planar photodiode and Indium doped n + n p hetero junction mesa photodiode formed in situ by molecular beam epitaxy for Mercury Cadmium Telluride long wavelength detector was compared and analyzed.It was found that n + n p hetero juction mesa photodiode doped in situ had higher zero bias resistance area product ( R 0A ) than n on p planar photodiode in our experiment. By fitting with experimental data, R 0A at different temperature and the dark current at different bias voltage of the two long wavelength devices were calculated theoretically, and some correlated parameters were also achieved.