The sliding in a Ni symmetrical tilt grain boundary, Sigma5 (012) [100], is investigated by modified Parrinello-Rahman molecular dynamics which deals with the thermodynamic ensemble characterized by the number of atoms N, temperature T, and the shear stress t. It is found that the grain boundary can slide in the direction perpendicular to the tilt axis [02(1) over bar] with the aid of shear stress in that direction. The sliding is coupled to the migration involving a collective motion of the third layer atoms hopping into the hollow site in between the first (GB) and second layers. Here, the critical stress necessary to move the grain boundary damps almost exponentially as the temperature is increased. At temperatures above about 820 K, random walk of GB migration occurs without the assistance of stress. On the other hand, it is only at much higher shear stress that the grain boundary can slide in the direction parallel to the tilt axis [001]. This may be because this sliding mechanism does not involve the coupling to the migration mode.