Two dielectric barrier discharge reactors, based on L-shaped electrodes, were designed and fabricated for in-line surface treatment of polyimide (PI) substrates. Under homogeneous discharge mode, their surface treatment performance was evaluated in terms of water contact angles (WCAs) of PI film. Hydrophilicity of polymer films arises from the grafting of functional groups onto the polymer's surface by O and N radicals. In our reactor, ambient air could be entrained into the discharge region, such that the WCA of PI film was effectively lowered without adding O-2 or N-2 gas. The PI film WCA decreased with increased applied voltage or with decreased motion speed of the substrate. A serial connection of the electrode lowered the WCA, whereas O-2 addition only weakly affected the PI film hydrophilicity. The mechanisms underlying these phenomena were analyzed using the discharge current, the temporal and time-averaged discharge images, and the optical emission profiles.