This paper signifies effect of nickel oxide nanoparticle doped biodiesel on CI engine performance at different fuel injection timing. The base fuel used (NBE25) is a blend of Azadirachta indica biodiesel (25%) and conventional diesel (75%). nickel oxide nanoparticle was doped in the NBE25 blend and four dosing levels of 25, 50, 75 and 100 ppm. The nanoparticle was prepared by homogenous addition method and various characterization techniques were carried out to verify the significance of structural and physical properties of nickel oxide. Experiments were conducted on DI-CI Tangentially Vertical Type VCR engine at three injection timings (IT) 23 degrees bTDC, 19 degrees bTDC and 27 degrees bTDC. Recordings revealed that advancing the fuel injection timing resulted in higher heat release rate and NOx levels. The average reduction in Brake specific fuel consumption for timing of 27 degrees bTDC compared to 23 degrees bTDC for dosing levels of 25 ppm, 50 ppm, 75 ppm and 100 ppm in base fuel were 5.29%, 6.91%, 7.13% and 7.86% respectively. The brake thermal efficiency for 27 degrees bTDC found to be higher compared to 23 degrees bTDC and presence of nanoparticles provided secondary atomization which therefore enhance complete combustion of fuel.