A computational analysis of nozzle flow characteristics and plume structure is conducted to examine performance of the supersonic nozzle employed in a thruster for ground firing test. At first, flow simulations in two-dimensional converging-diverging nozzle are performed for the verification of computational capability as well as turbulence model validity. Axisymmetric converging-diverging nozzles for ground firing test are analyzed with the k-${omega}$ SST model. A performance penalty caused by flow separation in a diverging section is observed in initially-designed nozzle. The performance could be enhanced by the modification of the diverging section of nozzle contour.