With taking account of alternation of kinetic coefficients, we study the possibility of step instabilities on a Si(0 0 1) vicinal face. In sublimation, a step with large kinetic coefficient recedes faster than that with small kinetic coefficient, and step pairs are formed. The upper side step in the step pair is the step with large kinetic coefficient. An equidistant array of the pairs is unstable against bunching. Number of steps N-max in bunches increases with time as N-max similar to t(beta). The exponent beta = 0.5 when the bunch grows via successive collisions of step pairs, and beta approximate to 1.2 when the bunch grows via coalescence of bunches. (C) 2007 Elsevier B.V. All rights reserved.