This study examines the consequences of concentration and flow rate of carbamide (urea) to reduce NO_x emissions from a diesel engine propelled with biodiesel. Taguchi-Grey relational-based multi-response optimisation method was employed for test-plan. Results revealed that carbamide flow rate and its concentration have a positive impact on NO_x emissions reduction in unmodified diesel engine. The obtained results are also justified with results attained from ANOVA. It is also confirmed that the results obtained in the experimentations are in line with the calculated values obtained Taguchi technique. Hence, the usage of carbamide in tailpipe is an effective way in reducing NO_x emissions from diesel engine.