The effects of fuel droplet size on soot formation in spray flames formed in a laminar counterflow are investigated experimentally and numerically. Sauter mean diameter (SMD) of quasi-monodispersed fuel spray (n-decane) is carefully controlled independently from the other spray characteristics using a frequency-tunable vibratory orifice atomizer, and the two-dimensional spatial distributions of soot volume fraction and soot particle size are measured by laser induced incandescence (LII) and time resolved LII (TIRE-LII), respectively. In addition, the soot formation processes are examined in detail by a two-dimensional direct numerical simulation (DNS) employing a kinetically based soot model with flamelet model. The results show that the soot formation area and location are strongly affected by the SMD of the fuel spray. As the SMD of the fuel spray increases, the average soot formation area expands, whereas local suppression of soot formation is instantaneously observed in the spray flames because of the appearance of groups of unburned droplets. The size of soot particles tend to be larger in the outer part of the soot formation area compared to soot in the inner part. This is because the surface growth of soot particles markedly proceeds compared to the coagulation and oxidation. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.