Surface treatment is one of the key issues for fabricating long-wavelength infrared (LWIR) detectors having good performance. In this paper, a novel surface treatment using hydrazine has been proposed for HgCdTe and its validity has been confirmed with ellipsometry and C-V analysis. The fixed charge density and trap oxide charge density of hydrazine-treated sample have shown so small value of 1.29×109 cm-2 and 4.35×1010 cm-2, respectively, comparing with those obtained with the conventional Br-MeOH-treated sample. In addition, the hydrazine-treated sample has shown high frequency characteristic in the C-V curve, which means the large effective minority carrier lifetime on the surface. By applying the new surface treatment using hydrazine to vacancy-doped wafers, LWIR photodiodes have been successfully fabricated. Current-voltage (I-V) characteristics of the hydrazine-treated Hg0.77Cd0.23Te diodes were also measured. Average R0A products of these diodes with the junction area of 30x30 μm2 were about 2.54 Ωcm2, which satisfy 95% BLIP (background limited infrared photodetector) condition for LWIR photodiodes.