Ti/Al contacts have been deposited by electron beam Evaporation onto N-polar n-type surfaces of GaN-based vertical structure LED on Si substrate. The effect of AlN buffer layer on ohmic contact of these chips has been investigated through I-V characteristic. The results shown Ti/Al contacts prepared on N-polar n-type surface without AlN buffer layer became ohmic contact after annealing in the temperature range of 500—600℃. The as-deposited Ti/Al contacts on N-polar n-type surface with AlN buffer layer shown ohmic behaviors with a specific contact resistivity of 2×10-5 Ωcm2 and maintained ohmic contact characteristics until anneal at 600℃. Therefore, The exsiting of AlN buffer layer is the key to forming highthermal stability ohmic contact for GaN-based vertical structure LED on Si substrate.