Abstract: The influence of counterweight blocks on dynamic balance of a fixed-pitch screw rotor vacuum pump was mathematically modeled, theoretically analyzed in rigid-body dynamics and numerically simulated with software MATLAB GUI. In the simulation, the unbalanced mass-radius products of rotor and counterweight blocks were decomposed into components in the two selected parallel planes; the mass, geometry and alignment of the counterweight blocks were optimized in such a way that the total mass-radius products in the selected planes were zero and a stable dynamic balance of the screw rotor was reached. Exemplified with a four-lead fixed-pitch screw rotor pump and experimentally tested with SolidWorks motion code, the results show that the counterweight method significantly improves the stability and keeps intact the structural integrity of the screw rotor vacuum pump.