A metal-oxide-semiconductor (MOS) capacitor was fabricated using 4H-SiC epitaxial layer, and the interface state was evaluated in oxygen and hydrogen ambient under high-temperature conditions by the AC conductance technique. The relationships between interface state density (Dit), and corresponding time constant (tit) were obtained. Influences of oxide thickness and of gate metal (Pt or Al) were studied. Dit of Pt gate capacitor is influenced by ambient gas at higher temperature but Dit of Al gate capacitor is little affected by ambient gas. Dit of capacitor with thicker oxide layer tends to be lower than that of capacitor with thinner oxide layer. Interface states with larger time constant are decreased for hydrogen ambient comparing with oxygen ambient in the Pt gate capacitor.