As one of the most complicated parts of an internal combustion Engine, cylinder head is directly exposed to high combustion pressures and temperatures. Cooling must be provided for the heated surfaces to avoid overheating. However over-cooling will cause lower overall efficiency and high emission. Therefore, an optimal design of the cooling system is required to maintain trouble-free operation of engine. For single cylinder naturally aspirated Compression Ignition (CI) engines, on account of space restrictions, designing of cooling jacket is very critical. Engineers invest a large amount of time and serious effort to optimize the flow through engine cooling jacket with limited detailed information of conducting flow and heat transfer.This paper therefore, investigates cooling performance of a single cylinder 510cc production diesel engine. Commercially available Computational Fluid Dynamics (CFD) code is used along with the domain knowledge of in-house experts to improve the cooling performance by virtue of better coolant flow distribution in head and cylinder barrel. Due to vehicle packaging requirements, coolant inlet and outlet are on same side of engine. This coupled with positions of flow connections between cylinder barrel and head resulted in coolant flow distribution such that 26% of total coolant flow was exiting from outlet without contributing to cylinder head cooling. This has lowered valve bridge maximum velocity to 0.8 m/s and restricted coolant flow rate in cavity below exhaust port to approximately 10% of total coolant flow.Coolant flow distribution was modified by changing diameter and position of coolant jacket connections between cylinder block and head to meet engine cooling as well as manufacturing requirements. Optimized coolant jacket reduced non-utilized coolant flow by 75% from baseline design leading to 35% improvement in valve bridge velocity. Coolant flow below exhaust port was improved by approximately 50%.Validation of CFD optimized cooling jacket was carried out by comparing coolant temperature in cavity below exhaust port.