Investigations are carried out to verify the performance and emission of a variable compression diesel engine operated with pure Neem oil Bio diesel (NeME), and pure Diesel at various compression ratios starting from 15:1, 17:1, 19:1 and 21:1 to compare the results at each compression ratio considering pure diesel as base line. The experiment has been conducted at fixed engine speed of 1500 rpm and engine tests have been conducted to get the comparative measures of Specific Fuel Consumption (SFC), Brake thermal efficiency (BTh) and emissions such as CO, CO 2 , HC, and NO to study the impact of pure NeME, with respect to Compression ratio and its implementation. A notable improvement in between NO and HC was observed maintaining a high thermal efficiency by a suitable combination of Neem biodiesel and compression ratio. The results of pure NeME operated under compression ratio 17:1 were found encouraging in all respects of performance and improved emission characteristics.