In this study, a vacuum assisted air gap membrane distillation process was designed to weaken the negative influence of non-condensable gases and improve the process performance. It was found that the applied vacuum (within a certain range of vacuum pressure) could remove the non-condensable gases and effectively improve the process performance. It was also found that water vapor cannot condense at the cooling surface in the module when the vacuum pressure is too high. Hence, the concept of optimal vacuum pressure was introduced and studied. The OVP was considered a critical factor to ensure that the VA-AGMD process was dominated by AGMD rather than by vacuum membrane distillation. The effect of some parameters on the optimal vacuum pressure was examined and is discussed in this paper. (C) 2017 Elsevier B.V. All rights reserved.