Diode-laser absorption computerized tomography is applied to the diagnosis of the plume generated by an arcjet thruster A protection stack is designed to carry a laser beam into the arcjet flow field at any location in the vacuum chamber. The experimental system is developed to reconstruct the maps of the temperature and the atomic number density by utilizing a small number of line-of-sight measurements. It has been demonstrated to be a convenient and useful method for the measurement of plume parameters and for monitoring the performance of the arcjet.