The formation of carbonaceous particulates in a co-flow laminar diffusion flame was studied using UV-visible spectroscopy and laser scattering/extinction techniques for measurements of volume fraction and particle sizes. Measurements were performed in a non-smoking ethylene-air flame at 1 atm. The mean size of nano organic carbon particles was ∼ 2-3 nm, which was in agreement with previous results obtained in rich premixed flames with equivalence ratios across the soot formation threshold. Nano-organic carbon particles were formed in the fuel side of the flame front closer to the flame center line than soot particles and with a concentration level comparable to that of soot suggesting a role mainly responsible for soot formation. The intermediate spatial location of these particles between the fuel- and soot-containing regions and their high concentration in flame suggested that soot formation is just the consequence of coagulation of NOC particles without a major role of surface growth in the soot loading process.