Metallization systems of Ti/Ni/Au and Ti/Pt/Au were deposited onto InAs/graded InGaAs/GaAs epitaxial layers grown on GaAs substrate. Both of them exhibit good nonalloyed specific contact resistance of 1.0 x 10(-6) and 3.0 x 10(-6) Ohm cm(2) for Ti/Pt/Au and Ti/Ni/Au metallization systems, respectively. However, the thermal stability can be achieved at least up to 350 degrees C for Ti/Pt/Au metallization system, while the Ti/Ni/Au metallization can only thermally stabilize at 250 degrees C. The degradation of the specific contact resistance at high annealing temperature is attributed to the induced decomposition of InAs and graded InGaAs layers. Rutherford backscattering spectroscopy spectra and Auger electron spectroscopy depth profile were used to study the physical mechanism.