Silicon carbide (SiC) single crystals, a promising material for high power and high temperature semiconductor devices, have microscopic hollow defects, so-called "micropipes." Their formation mechanism is not satisfactorily clarified yet. In this paper, a surface step model for micropipe formation in SiC single crystals is proposed, where the strong repulsive interaction between surface steps on the SiC(0 0 0 1) surface is a major driving force for coalescing unit cell size screw dislocations.