The role of positive Q-value neutron transfers in sub-barrier fusion reactions has been studied with a modified quantum coupled channels model with all order couplings (CCFULL model).Neutron rearrangement related only to the dynamical matching condition with no free parameters is implemented in the model,which provides a way to understand especially the Q-value dependence of sub-barrier fusion reactions.The fusion cross sections of the collision systems 40Ca+94,96Zr have been calculated and analyzed.The general trend of experimental data can be reproduced well with additional channels for neutron rearrangement.We find that enhancement of sub-barrier fusion cross sections is closely related to the Q-value of the transferred neutrons,in particular for channels with sequential even number transferred neutrons.