The cooling system design for a two-stroke, opposed-piston (OP) engine is substantially different from that of a conventional four-stroke engine as the opposed-piston engine requires efficient cooling at the center of the cylinder where the heat load is highly concentrated. A thermally efficient design ensures engine durability by preserving the oil film at the top ring reversal zone. This is achieved by limiting the surface temperature of the liner to below 270°C at this location. Various water jacket designs have been analyzed with computational fluid dynamics (CFD) using a "discretized" Nusselt number approach for the gas side heat flux prediction. With this method, heat transfer coefficients are computed locally given the flow field of the combustion gases near the liner surface and then multiplied by the local gas/liner temperature difference to generate the heat flux distribution into the cylinder liner. The heat flux is then averaged over the cycle before being applied as a boundary condition to the CFD simulation. The baseline design consists of a simple water jacket with coolant flowing axially from the inlet near the intake port to the outlet near the exhaust port. This approach yields uneven cooling both longitudinally and circumferentially about the cylinder liner. A greatly improved thermal response has been achieved by introducing the coolant at the hot center section of the liner with roughly half of the coolant flowing toward either end of the cylinder. A detailed analysis shows that liner surface temperatures well below 270°C can be achieved for an engine with a power density of 50 kW/liter by carefully optimizing the coolant velocities in the center section of the liner.