Modeling studies are conducted to investigate the characteristic of heat transfer and flow in low and medium power arcjet thrusters with hydrogen as the propellant. It shows that low and medium power arcjet thrusters share some common features for the flow and heat transfer. The highest plasma temperature appears at the location near the cathode tip due to the arc heating in the near-cathode region. And the maximum velocity appears in the downstream of the constrictor. With the increase of the power, the maximum temperature and velocity within the thruster increase. Modeling results are found to compare favorably with available experimental data for the arcjet thrusters with different power levels.