The lateral flow force acting on the spool was analyzed by numerical simulations of three-dimensional flow. The test spool valve was the practical hydraulic type having non-axisymmetrical flow paths with inlet and outlet ports near the valve orifices. In case of the inflow jet, lateral flow force was found to depend on the jet impinging pattern such as jet angle and jet impinging point in the azimuthal direction on the surface of spool. The sum of the lateral forces is very small because the forces acting on the surface at the different axial positions counteract each other. In case of the outflow jet, the force is dependent on the jet impinging pattern as well as the flow pattern near the outlet port edge. The sum of the forces acting on the sorface in the same direction increases with decreasing the spool stroke.