Based on the self-similarity of droplet breakup in the secondary atomization region, the atomization process of coaxial air-blast atomizer has been investigated. The relationship of Sauter mean diameter (SMD) with the effects of gas jet velocity, liquid jet velocity and diameter and liquid/gas mass flux ratio was obtained according to the breakup time and motion characteristic of droplet in air stream. The four parameters in the relationship of SMD were estimated according to the experimental results of nine coaxial two-fluid air-blast atomizers with air and water. The results showed that the obtained relationship of SMD can be used to predict the SMD of coaxial air-blast atomizer when m > 13.99d(1)(-0.1285)d(g)(-0.9415). (C) 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.