Effect of Cu segregation on surface flaws of CSP hot strips has been studied. Based on the Copper segregation equilibrium, it is concluded that the surface flaws can be reduced by controlling the content of Cu in liquid steel, and the content of other elements of low melting point. Increasing the soaking temperature is also helpful. The Copper sulfide precipitates in nanometers size. Influence of Cu on quality of the CSP hot strips is discussed.