Comprehensive dissipative particle dynamics simulations are performed to investigate the effect of A-block polydispersity on the phase behavior of AB-diblock copolymers. The experimental results of Lynd and Hillmyer on polydispersity induced domain spacing expansion at different segregations (Macromolecules 2005, 38, 8803) are well reproduced and explained in terms of interfacial free energy and molecular stretching in domain center. Short block copolymers in polydisperse system are found to preferably accumulate at A/B interface as compatibilizers causing a reduction in interfacial free energy. At the same time, domain centers are found mainly occupied by long blocks which in turn determining the spacing of the corresponding domain. More importantly, increase in polydispersity will introduce more long blocks into system and therefore will enhance the molecular packing and stretching in the domain center. Such chain stretching in domain center and reduction in interfacial free energy are found to be more severe at stronger segregation, which will lead to more severe domain spacing expansion with increasing polydispersity. (C) 2013 Elsevier Ltd. All rights reserved.