In this study, the performance of regenerative cooling system for large expansion ratio rocket engines (Ae/At ∼ 100) is investigated numerically. During combustion and gas expansion, the walls of the combustion chamber and the rocket nozzle are exposed to high temperature gas (∼3500 K), which can ultimately lead to structural failure. Therefore, to protect the hardware from thermal failure, a regenerative cooling system for a cryogenic rocket engine that uses fuel (liquid hydrogen (LH)) or oxidizer (liquid oxygen (LOX)) as the cooling medium is considered. Three-dimensional simulations have been performed for both constant and variable fluid properties. The influence of the thermal properties of the material and thickness of the nozzle wall on conductive heat transfer has also been investigated. The effect of radiative heat transfer when there is no regenerative cooling system has been analyzed. In addition, heat transfer enhancement for different turbulence models and the influence of coolant used (both the fuel and oxidizer) is also investigated. It is evident from the results that a properly designed regenerative cooling system can maintain the hot side wall at a temperature well below the melting point of the wall material, which ensures the protection of nozzle hardware from thermal failure. Also, the predicted pressure drop is found to be 0.7 bar, which meets the design requirement. Numerical predictions are validated with the data available in literature.