We study the 1/f noise currents and dark currents in LWIR HgCdTe photodiodes with different passivation. The diodes are fabricated by ion implanting boron on MBE HgCdTe with x=0.2173. One kind of photodiodes was passivated by ZnS and the other kind was passivated by CdTe/ZnS. Both dark currents and 1/f noise currents were measured at several reverse bias voltages. The measured dark currents of the photodiodes are analyzed using current model fitting methods. The different dark current components, such as diffusion current, generation-recombination current, trap assisted tunneling current and band-to-band tunneling current, at various biases voltages can be separated from the measured dark currents. The measurement results demonstrate that the dominant mechanism that produces 1/f noise in HgCdTe photodiodes with either passivation is tunneling. When the reverse bias voltages are less than 200mv, the main mechanism that produces 1/f noise is trap assisted tunneling. In this case, the 1/f noise currents of the photodiodes passivated by ZnS are smaller than those passivated by CdTe/ZnS. When the reverse biases are larger than 200mv, the band-to-band tunneling currents of the photodiodes passivated by ZnS are much larger than the photodiodes passivated by CdTe/ZnS. And the 1/f noise currents of the ZnS passivated photodiodes are larger than the different passivated one. In order to investigate the effect of surface passivation on the stability of two kinds of diodes, R-V characteristics and 1/f noise of the diodes were measured after vacuum baking for 10 hour at 80°C, the photodiodes passivated by CdTe/ZnS show higher performance compared with the diodes passivated by ZnS after baking.