A new method, "nonvolatile solvent vapor annealing" (NVASA), has been developed to anneal block copolymers during film deposition by controlling the solvent drying process. Precise amounts of high boiling point additive added to the polymer solution briefly remain in the polymer film after casting, leaving the film in a swollen state, increasing its chain mobility, and ultimately improving domain order. We demonstrated the effectiveness of NVASA on several block copolymer systems and used in situ grazing incidence small-angle X-ray scattering (GISAXS) to validate the ordering process during the self-assembly. The simplicity and reproducibility of the method is attractive for implementation in large-scale manufacturing processes such as roll-to-roll printing as swell ratio is easily controlled by the amount of additive used and separate annealing steps are not needed. This work potentially introduces a new way to quickly and cost effectively anneal block copolymers.