Testing the performance of the GMR sensors are accomplished by providing electromagnetic field variations and measure the change of sensor resistance as a response of the input. The program, created using LabView, can be used to design the magnetic circuit to generate the electromagnetic fields. The Input parameters are relative permeability of core, the number of turn, core dimensions, fringing and air gap. While the output parameters are core reluctance, air gap reluctance, flux in the core, flux on the air and electrical current required. Input parameters are calculated using the electromagnetic equations to obtain the output. Manufacture of magnetic circuit as a source of electromagnetic fields for testing of GMR sensors and determination of accurate physical parameters become easier with this program.