The paper presents an experimental investigation of the effect of oxygen concentration in the oxidizer (O-2+N-2) on distributions of soot volume fraction (SVF) and flame centerline primary particle size in laminar coflow ethylene diffusion flames. The base flame has an oxygen index (OI, oxygen mole fraction in the oxidizer stream) of 21%. Three additional flames, with OI of 30%, 40% and 50%, are also examined. The fuel flow rate was maintained constant in all the experiments. The two-dimensional laser-induced incandescence (2D-LII) and two-color spectral band measurement based on the response spectrum of R, and G bands of a color 3-CCD camera are applied to measure distributions of SVF. The time resolved laser-induced incandescence (TIRE-LII) is used to measure SVF and primary particle size along the flame centerline at different heights above burner (HAS). The results of SVF are compared among the two-color method, 2D-LII, and light-of-sight attenuation (LOSA) from the literature in the base flame and compared between TIRE-LII and two-color spectral band method in oxygen enriched flames. Good agreements are observed in the measured SVF from different methods. Soot primary particle sizes, measured by TIRE-LII, are compared with those based on transmission electron microscopy (TEM) image analysis from the literature and a fairly good agreement is observed. The results indicate that soot yields increase and the flame becomes shorter and brighter with increasing OI. The primary particle diameter increases firstly and then decreases with the normalized HAB by the visible flame height (h(f)). At the same HAB/h(f), the primary particle diameter generally increases with increasing OI from 21% up to 50%. Crown Copyright (C) 2019 Published by Elsevier Ltd. All rights reserved.