A hydraulic pump includes an upper housing with fluid inlet and outlet ports, and a lower housing combined with the upper housing. A driving shaft is inserted into the upper and lower housings such that the driving shaft is rotated. A cylinder block is mounted within the upper housing around the driving shaft such that the cylinder block is rotated together with the driving shaft. Uneven numbers of pistons are equidistantly arranged in the cylinder block such that the pistons reciprocate. Each piston has a ball bearing. A swash plate member is provided at the lower housing. The swash plate member has a spiral groove for receiving the ball bearing of the piston. The spiral groove of the swash plate member incurs the stroke of the piston. The spiral groove of the swash plate member has a left spiraled portion and a right spiraled portion interconnected by curved portions.
