Electromagnetic stiffness is an important parameter of stepper motor. It impacts obviously on system natural frequency. In this paper, the torsional vibration mechanism of stepper motor rotor caused by electromagnetic stiffness is studied and the mathematical formula for electromagnetic stiffness is established derived from stepper motor mathematical models. We simulate and experiment on stepper motor with parameters in the paper. Simulation value and experiment value of the electromagnetic stiffness are consistence with the numerical one. So it verifies the existence of electromagnetic stiffness, which provides a basis for researching on frequency variation of motor dragging flexible load.