Compression ignition (CI) engines contribute more detrimental emissions when compared to petrol engines, especially NOx and smoke emissions. The main reason for diesel engines to emit higher NOx emissions is owing to higher excess air ratios, which promotes the NO formation chemistry and thus emitting marginally lowered HC and CO emissions. Biodiesel applications are becoming very important and gaining rapid momentum in the arena of depleting fossil fuels and increasing pollution norms. Biodiesel-fuelled CI engine gives relatively better performance and reducing emissions; however it increases the nitrogen oxides (NO_x) emissions. This review paper studies on NO_x emission reduction techniques such as water injection, emulsification, injection timing retardation and simultaneous technology and its effects on various operating parameters carried out in biodiesel powered CI engines. The emission characteristics of the diverse biodiesel-diesel blends have been summarised and discussed. NO_x emissions can be reduced up to 37–50% by water injection method in biodiesel engines. It is also inferred that water-biodiesel emulsification reduces the NO_x about 10–60% compared to conventional diesel. However, all these technologies reduce nitrogen oxides and particulate matter emissions considerably but increase HC, CO emissions and increase specific fuel consumption (BSFC) thereby reduces brake thermal efficiency (BTE).