The flow inside a laser thermal thruster was analyzed to estimate the conversion from the laser beam energy to the enthalpy of a fluid. Since the energy conversion process occurs inside the Laser-Sustained Plasma (LSP), interest is focused on analyzing the LSP, in which the chemical, radiation, convection, dissipation processes strongly couple each other. Calculations showed that the plasma was sustained at lower temperature in the air than the nitrogen. The absorption of the laser beam power could be increased by changing the focus position, however, most of it was cancelled by the increased radiation from the LSP.