Recently, forward osmosis (FO) has received much attention as an advanced water treatment technology. Although the FO process has begun to spread widely worldwide, it still has some problems - such as the low water permeation rate compared with those required for some applications - that must be solved for its commercial application. To achieve the higher water flux and lower reverse salt flux, pressure assisted osmosis (PAO), in which pressure is applied to a feed solution (FS), has recently been proposed. In this work, experiments were carried out to investigate membrane structures and operating conditions in the PAO process by using three types of cellulose triacetate (CTA) - hollow fiber (HF) membranes. The HF membranes are preferable in the FO process because a high packing density and a large specific surface area can be obtained in the HF module. In addition, the HF membrane module has four ports and does not require a spacer between the membranes. The effects of the applied pressure, the draw solution concentration, and the structure parameter of the HF membranes on the PAO performance were investigated. The water flux in the PAO process was theoretically analyzed. The calculated results satisfactorily agreed with experimental data. (C) 2015 Elsevier B.V. All rights reserved.