Abstract: A small thruster plume flow of nitrogen at high altitude is numerically investigated by using a DSMC method. A hard sphere model is employed to simulate nitrogen molecule. The collision frequency is calculated through using the random sampling frequency method for each cell. An axisymmetrc model based on a two-dimensional modification is introduced to model the flow field. To raise the computation efficiency of the DSMC method, both a grid domain decomposition method and a local grid clustering technique are enforced. An inner flow numerical result is considered as the outflow conditions of the thruster, a escaping boundary condition is implemented at far-field boundary, and a perfect scatter reflection model is chosen as the solid wall boundary condition. Moreover, molecule rotation energy included in inner energy is considered besides translation energy. The reliability of the numerical method is demonstrated by comparing with the relevant experimental results. Both the properties of plume flow at high altitude and the nonequilibrium effect related to rarefied gases are analyzed.Resultsshow that the DSMC method coupled with a fine grid technique is an effective approach for high altitude plume simulations, and that the nonequilibrium effect of gases must be numerically dealt with. Moreover, pitot pressures are given numerially in good agreement with measured data.