Abstract: A special calculating rule (P-Q rule) is introduced in this paper for the selection of optimum parameters in power mechanism of electro-hydrau servo-system. With the aid of this rule, it is possible to determine the no-load flow of the servo-valve and effective area A of oil-cylinder piston with one of four optimum indexes as following: 1. Minimum power required; 2. Minimum A(or D)(fast response of system); 3. Qm and A at relative minimum (sub-optimization); 4. Judgement of applicability of given servo-valve and selection of minimum A with given servo-valve. The special features of this calculating method is that the load locus can be of any geometric shapes and the piston areas of the two ends of oil-cylinder can be equal or unequal. The methods in the past are only applicabe for special load locuses, just like straight-line, normal ellipse etc as well, the piston areas of the two ends of oil-cylinder must be equal.