We have investigated the thermal stability of three composite metals on their contact resistivities and luminous intensities for using as the reflector in flip-chip light-emitting diode (FCLED). The composite metals were simultaneously deposited on n-type GaN without alloy to form n-type Ohmic contact and simplify the process. The investigated composite metals were Ti/Al/Ti/Au (30/500/30/300 nm), Cr/Al/Cr/Au (30/500/30/300 nm) and Cr/Ti/Au (500/30/300 nm), respectively. The specific contact resistivity of Ti/Al/Ti/Au, Cr/Al/Cr/Au and Cr/Ti/Au on the n-type GaN Ohmic contact were changed from 5.4×10-4, 6.6×10-4 and 7.7×10-4 Ω-cm2 to 5.3×10-4, 4.5×10-4 and 1.3×10-4 Ω-cm2 respectively after 500 hours thermal stress at 150°C in the air. After 96 hours of thermal stress, the luminous intensities at 20 mA of these three structures were decreased 6.2%, 11.1% and 1.4%, respectively. Therefore, in addition to maintain good n-type ohmic contact and simplify the process, the Cr/Ti/Au composite metal demonstrates good thermal stability as a reflector in FCLED.