In this paper, we proposed the modified method of Metcalfe's law that is "Modfied law" in evaluating the network power for the measurement of combat effectiveness in C4I system. It is applied to JFOS-K (Joint Fire Operating System-Korea) C4I system that can connect KJCCS (Korea Joint Command and Control System) of Korea armed forces with JADOCS (Joint Automated Deep Operations Coordination System) of U.S. armed forces and achieve sensor to shooter system in real time in JCS (Joint Chiefs of Staff) level. The result of combat effectiveness using Modified law is compared to the one by C2 theory and found that both is similar. This study is meaningful because we improved the description level of reality in calculation of combat effectiveness in C4I system.