Abstract: A simplified physical model of an arcjet thruster was established on the basis of proper understanding of the physical mechanism of its working course, and a mathematical model for it was expressed by using Navier-Stokes equations coupled with electromagnetic source terms based upon the physical model. Calculation for working performance of a low power hydrazine arcjet thruster with arc and without arc was achieved comparatively. Second-order-accuracy non-oscillatory and non-free-parameters dissipative finite difference scheme and explicit time-marching method were used to obtain numerical solutions. The computationalresults present flow configurations within the thruster with arc and without arc, foretell its thrust and specific impulse in various conditions, and attest its working exchangeability.