Screw dislocations near a subsurface crack have been investigated. Based on the Schwarz-Christoffel transformation, the system was mapped to that of a screw dislocation near a free surface. The complex potential, stress field, stress intensity factor, dislocation image force and strain energy were obtained. The effect of the subsurface layer between the crack surface and the free surface on the stress intensity factor, image force and strain energy was studied.