A hydraulic breaker assembly including a head cap, a cylinder, and a front head, which are mounted in a casing, wherein corresponding faces of the front head and the cylinder are connected by connecting pins, wherein the cylinder and the head cap located above the cylinder are fixed together by fastening bolts, and wherein the front head, the cylinder, and the head cap are supported on an inner side of the casing by variable dampers, which are brought into close contact with the front head, the cylinder, and the head cap. The movement in the casing is thus prevented so that the head cap, the cylinder, and the front head are strongly coupled together. The length of the fastening bolt is reduced to minimize the impact-generating section so that the bolt is protected from distortion stress occurring upon striking of the rod, thereby increasing durability.