Our theoretical study of step bunching induced by the drift of adatoms is reviewed. The theory may apply to the bunching in Si(111) vicinal faces with direct current heating. In the occurrence of evaporation of adatoms (nonconserved system), a vicinal face is unstable when the velocity of the down-hill drift exceeds a critical value. After the instability, the terrace width L grows as L∼t^β with the value of β smaller than 1/2 in an early stage, and saturates if the drift velocity is small. The saturation of L is expected from the Benney eyuation, which governs the surface profile near the threshold of instability. With increasing drift velocity the bunch size grows via coalescence and the exponent β approaches 1/2. When the evaporation is neglected (conserved system), a vicinal face is always unstable with the drift. The terrace width grows by coalescence of bunches and f ti β≈1/2 independent of the form of the step repulsion.