The fabrication process of InGaAs mesa detectors contains the ohmic contacts of n-InP and the stretched electrode with good reliability. Cr/Au as the contact material for the n-InP is reported. The transmission line method (TLM) is employed to research the ohmic contacts at different annealing condition and Auger energy spectra (AES) is employed to research interface between Cr/Au and n-InP at different annealing condition. The results show that the good ohmic contacts are obtained without annealing treatment, while the ohmic contacts are degraded after annealing. The presence of Cr2O3 in the Cr layer deposited with ion beam sputter causes oxygen out-diffusion into the Au layer at annealing condition. The fabrication process of InGaAs mesa detectors is simplified by using the unannealing Cr/Au as the ohmic contacts of n-InP and stretched electrode.