In order to study the dark current of the devices, in this paper, the dark current of In0.83Ga0.17As p-i-n photodiodes was analyzed. Extended wavelength In0.83Ga0.17As p-i-n photodiodes with mesa type configuration were fabricated by two different processes. The first process(device marked M135L-5) was: rapid thermal annealing (RTA) was performed after mesa etching. The second process(device marked M135L-3) was: RTA was performed before mesa etching. Dark current mechanisms for extended wavelength In0.83Ga0.17As p-i-n photodiodes with different device fabrication processes were studied by means of the current-voltage curves at different temperatures and bias voltages. In contrast to M135L-5, M135L-3 had a lower dark current at the same test temperature from 220 K to 300 K. The ratio of perimeter-to-area(P/A) was used to characterize the perimeter-dependent leakage current and the area-dependent leakage current. The results show that M135L-3 has a lower area-dependent leakage current. Activation energy of devices served as a method to estimate the dark current composition was extracted from current-voltage curves. The results indicate that the dark current of M135L-5 is dominated by diffusion current at reverse 0.01-0.5 V bias voltage and at 220-270 K. The dark current of M135L-3 is dominated by diffusion current at 250-300 K as well as dominated by generation recombination current and surface recombination current at reverse 0.01-0.5 V bias voltage and at 220-240 K. Meanwhile, the results of dark current fitting also show the same conclusions. The studies have shown that M135L-3 with annealing treatment and optimization process is better than M135L-5 for reducing dark current because the RTA decrease the bulk dark current.