By using atomistic simulation, we study motions of dislocation and low-angle grain boundaries with various misorientation angle in two-dimensional nanosheet. During dislocation motion under externally applied stress, the internal stress is significantly dropped and its magnitude can be analyticaly computed by lattice dynamics theory. The magnitude of stress-drop is closely related to speed of dislocation and external conditions. This stress-drop phenomenon is also observed during grain boundary motion, which consists of several number of dislocations and the magnitude of stress-drop is much larger compared to dislocation case. Especially, we prove that the applied stress acts as a wave propagating to inside of nanoplate, which induces a curvature of the grain boundary. We investigate the effect of the wave propagation by tracking displacement of each atom and each structure unit which compose the grain boundary.