The intention of this review study is to investigate the control measures of NO_x emission. Even though the diesel engine sector plays a pivotal role in economic growth, it brings an unavoidable environmental deterioration. The current work reviews the effect of EGR, boost pressure variation, fuel modification, advanced engine combustion concepts, and after treatment processes for reducing NO_x emission to EURO VI level. The outcomes of bio-diesel performance and other emission characteristics are too highlighted. In addition to this, specific attention is given to Porous Medium Combustion (PMC) technology adopted in diesel engines and a separate section is devoted. In order to facilitate a better understanding of PMC technology, simulation and experimental results of NO_x emission are reported. Through this porous medium experimental study, it was found that NO_x, HC and CO emissions were reduced significantly. Reducing the NO_x and PM emissions might simultaneously increase the chance of usage of diesel and biodiesel blends fuelled CI engines in near future. The findings of this research work serve as best possible ways for achieving ultra-low NO_x emission subjected to EURO VI norms level is suggested in future directions to give the reader insight and the track for the future investigation.