Polyethersulfone/sulfonated polyetheretherketone (PES/sPEEK) supported osmotic thin film composite (TFC) membranes were fabricated with a focus on the optimization of the support layer of the TFC membrane by the phase inversion process. The polymer solution was cast directly on a glass substrate without any backing fabric to prepare the support layer. All resultant supports showed a fingerlike pore structure. The addition of a more hydrophilic polymer (sPEEK) to the polymer solution induced pores in the bottom layer of the support with average pore diameters ranging from 0.07 to 0.30 gm, which improved water flux. Bottom surface porosities were simply controlled by adjusting the polymer blend ratios. FO experiments were carried out in a high throughput (HT) crossflow FO cell with DI water and a 0.5 M NaCl solution as feed and draw solution, respectively. TFC membranes with 5% sPEEK showed a water flux in PRO mode of 14.3 LMH and in FO mode of 6.2 LMH, which is superior to the commercial cellulose triacetate (CTA) and TFC membranes (HTI). In addition, a mechanism for the bottom layer formation was proposed.