Propulsion system using continuous-wave (CW) CO2 laser has several advantages in comparison with other systems. This system does not require strong structure compared with propulsion with chemical propellant, plasma and gas arc jet. Laser propulsion can produce high specific impulse which determines propellant efficiency since the highest energy density is limited simply by laser system capability. Furthermore, propellant selection is free from the combination of fuel and oxidizer and therefore can be made according to propulsion performance. Using system of water vapor and CO2 laser which can offer large power system and has large absorption coefficient with water molecule, we consider rocket lunching from the ground to the earth orbits. On the other hand, laser propulsion uses high temperature propellant compared to the chemical rocket and gets high specific impulse under subsonic speed. We examine the performance of propellant that is heated with laser from downstream of the flow and estimate specific impulse and thrust using numerical calculations. We found that specific impulse can be increased by selecting proper combination of laser power and propellant mass flux.