We present an ab initio study of the phase transition cubic diamond ->beta-tin in Si and Ge under hydrostatic and nonhydrostatic pressure. For this purpose we have developed a method to calculate the influence of nonhydrostatic pressure components not only on the transition pressure but also on the enthalpy barriers between the phases. The calculations were performed using the plane-wave pseudopotential approach to the density-functional theory within the local-density and the generalized-gradient approximation implemented in VASP. We find good agreement with available experimental and other theoretical data.