The relative stability between wurtzite and zinc-blende structures in semiconductors is systematically investigated using a simple formula for calculating the energy difference between them. Using the well-known values of ionicity and lattice parameter for semiconductors, the energy formula gives good estimates of thc energy difference for group IV, III-V and II-VI semiconductors. The calculated energy differences agree well with those obtained by ab initio calculations ill thr literature. A simple criterion for wurtzite-zinc-blende polytypes in semiconductors is extracted in terms of ionicity from the formula. The criterion is also applied to the structural stability of GaN at various hetero-interfaces.