Geant4 is an Object-Oriented toolkit for simulating the interaction of particles with matter. It provides an abundant set of physics models which describe various kinds of interactions. EGS4 is one of the most well-known simulation packages for electromagnetic physics and widely used particularly in medical research area. We have already succeeded in interfacing EGS4 with Geant4, and Geant4 powerful resources such as geometry, tracking etc. can be shared on the Geant4 system, providing a common environment for comparison tests in users' application level. On the other hand, a project is in progress for systematic and quantitative validation of Geant4 electromagnetic physics comparing with well-established reference data in the medical physics domain (NIST and ICRU databases). In this paper, we combine these activities on the Geant4 system, and perform precision validation of two simulation codes with respect to protocol data, showing a well-working systematic way of evaluation of different simulation codes. To establish such methodology for comparing different simulation systems is quite important for critical application like medical physics. © 2004 IEEE.