A hydraulic proportional compound valve, which controls flow direction and flow rate as well as pressure, has characteristics such that the flow rate is proportional to the input current. Thus, the pulling force exerted by the solenoid valve in a compound valve should be constant over the specified range of the stroke of plunger. The design condition of a small d.c. solenoid valve which could satisfy the required characteristics is investigated analytically and experimentally. An analytical method predicting magnetic force is presented using the concept of magnetic force along the stroke is presented. It is shown that constant plunger force can be obtained providing a nomagnetic material inserted in the yoke of the solenoid and that the clearance between the yoke and the plunger is also an important factor for constant plunger force. It is found in the experiment that silicon steel as a yoke material is better than pure iron in magnetic hysterisis, while pulling force of silicon steel yoke is less than that of pure iron yoke. The comparison of analytical results with the experimental data shows satisfactory agreement.