The behavior of an extended dislocation near a semi-infinite crack is investigated. The criterion for the determination of equilibrium separation between Shockley partials and the necessary condition for the formztion of extended dislocations by dissociation reaction are developed. It is found that the equilibrium separation increases as the extended dislocation gets closer to the crack. If the stacking fault energy is comparable to the critical energy for dissociation in a perfect medium, complete dislocations can dissociate', to form extended dislocations near the crack. For an extended edge (or screw) dislocation near a crack, it produces not only the primary shielding/antishielding effect on modes I and II (or mode III) of fracture but the secondary effect on mode III (or modes I and II) of fracture. The shielding/ antishielding effect on fracture is closely related to the stacking fault energy ~, of materials. With decreasing 7 the primary effect decreases, but the opposite for the secondary effect. In addition, if the extended dislocation slips on a plane containing the crack tip and inclined to the crack plane at an angle 0, the maximum shielding effect occurs at about 0 = 70 ยฐ for mode I of fracture and 0 = 0 ยฐ for modes 1I and III of fracture when the dislocation remains a constant distance from the crack tip.