The precipitation of gamma' phase in a commercial single crystal Ni-based superalloy with different cooling rates has been investigated by atom probe tomography. Numerous irregular interconnected gamma' precipitates in the size range of similar to 30 to 50 nm were obtained even utilizing the fastest possible cooling rate. Diffuse gamma/gamma' interface and far from equilibrium composition of gamma' phase were observed in the fast-cooled sample, suggesting that gamma' precipitation occurs via a spinodal decomposition at the very early stage.