A moving screw dislocation near a finite crack was investigated using dislocation modelling. Motion directions parallel (x direction) and perpendicular (y direction) to the crack surface were considered. After obtaining the dislocation distribution in the crack, we calculated the stress field in the medium. The contours of the stress component sigma _yz near a screw dislocation are squeezed along the direction of motion (i.e. the x direction) of the dislocation and those of sigma _xz are spread out laterally. Relative to the static screw dislocation, the magnitude of sigma _yz due to a moving screw dislocation in an infinite medium decreases with increasing velocity V_x. Hence the interaction of crack and dislocation is reduced if the velocity V_x increases. Generally, the screening effect of dislocation on fracture is reduced if the velocity V_x increases. The effect of velocity along the y direction on the stress intensity factor has the opposite trend to that along the x direction.