The effect of carbon addition on the solidification microstructure of the first generation single crystal Ni-based superalloy AM3 was studied. The results indicate that with increasing of carbon content, dendrite morphology and dendrite arm spacings have no obvious change, volume fraction of eutectic is significantly decreased, and the primary carbide is gradually increased. The primary carbide morphologies in this alloy commonly are blocky, framework-like and Chinese-script-like. When the carbon level is high, the carbide morphology becomes net-like by frame-like carbides connecting each other (Chinese script morphology).