Thermally stable, low-resistance PdGeTiAu ohmic contacts to high-low doped n-GaAs have been developed. The lowest contact resistance obtained is two times lower than that of previously reported PdGe ohmic contacts. The contacts are thermally stable even after isothermal annealing for 5h at 40$0^{circ}C$ under atmosphere ambient. X-ray diffraction rseults, Auger depth profiles, and crosssectional transmission electron microscopy show that the good PdGeTiAu ohmic contacst are due to the formation of both AuGa and TiO compounds. The AuGa compound enhances the creation of more Ga vacancies, followed by incorporation of Ge into Ga vacancies and TiO compound suppresses As outdiffusion from GaAs substrate, rspectively.