This paper analyzes the CMRR for a BME current mode instrumentation amplifier (CMIA) using operational amplifier power supply current sensing technique. A set of equations are derived for the case when all op amps have finite differential mode and common mode gains. In low frequency band where biopotential signals are usually located, better CMRR is achieved relying on better matching of two input op amps including their differential mode and common mode gains and also CMRRs. CMRR in three op amp IA is briefly reviewed and compared with CMIA. Simulation shows the results of theoretical analysis