We present a double-tube once-through steam generator (DOTSG) with an outer straight tube and an inner helical tube in this paper. For the purpose of the compact structure and lower pressure drop, the inner helical tube is optimized. The tube length and the combined pressure drop are considered as the two objective functions, and the pitch of the inner helical tube is considered as a design parameter. The multi-objective optimization model is established and translated into a single-objective function by objective product method (OPM). The single-objective function is optimized by fmincon routine in Matlab. The optimal pitches of the helical tube vary in sub-cooled region, boiling region and superheated region. The results show that the smaller pitch brings shorter tube length and higher pressure drop, and the effects are strong in sub-cooled region and superheated region, but weak in the boiling region. (C) 2012 Elsevier Ltd. All rights reserved.