Abstract : Considering the dangerous effects of ultrafine particles and air quality concerns worldwide, this work investigates the possibility of identifying operating criterion over which there is a greater tendency of formation of nanoparticle emissions in Diesel engines. The objective of this analysis is to study the emission of particles in the size range of 10 nm to 700 nm by subjecting a diesel engine to free acceleration tests. Details on the emission characteristics such as Particulate number, Particle mass, Particle diameter and the Lung deposition surface area in the size range 10–700 nm during specific conditions of acceleration and deceleration has been studied in this work. Maximum effect of the particles occurs during rapid acceleration. At idle speed, the Particle number, particle mass and Lung deposition surface area are very low indicating that the nanoparticle formation is low at idle condition. With increase in Particulate number, there is an increase in the Particulate mass and the LDSA. The particulate diameter is distributed across the PN range.