In order to remove and recover urea from its dilute aqueous solutions, e.g., wastewater drained from urea synthesis plants, continuous-effect membrane distillation (CEMD) process providing an extremely high thermal efficiency was investigated to concentrate aqueous solution containing urea. By using an air-gap membrane distillation module fabricated with both porous hydrophobic hollow fibers and dense-wall hollow fibers, the CEMD operation-mode can easily realize the direct recovery of condensation heat of the permeate to preheat the cold feed in situ within a single module. A series of experiments were conducted to evaluate the performance of CEMD process in terms of permeation flux (J(w)), gain output ratio (GOR) and rejection factor (R-f) under various operating conditions, such as the feed-in concentration, cold feed-in temperature, heated feed-in temperature and feed flow rate. The maximum values of GOR = 14.0, J(w) = 6.8 Lh(-1) m(-2) and R-f = 99.99% were achieved respectively. When a feed solution of 1.0 wt% urea was used as a sample wastewater, the feed could be at least concentrated to 40 wt% with high values of GOR, J(w) and R-f The experimental data showed that the mild operation condition could also help to avoid the urea loss from its thermal hydrolysis. A brief theoretical model was introduced and used to discuss experimental results. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.