Interfacial polymerization has been used for various applications because it can synthesize a nanometer scale selective layers via a solution process. The performance of membrane depends on the type of reactive monomers as well as reaction conditions. This study focuses on how the viscosity of organic phase affects the performance of the corresponding thin film composite (TFC) membrane. To investigate the effect of viscosity, three different types of organic solvents including n-hexane, light/heavy mineral oils were used. Trimesoyl chloride (TMC) and m-phenylenediamine (MPD) were selected for monomers in the organic and the aqueous phase, respectively, to produce TFC membranes for desalination. The membrane was characterized by measuring the water permeability and the salt rejection by using 0.2 wt% NaCl feed solution at 15 bar and 25 °C. The highest salt rejection was achieved by using a TMC/MPD ratio of 1:10 w/w%. At this concentration ratio, the water permeability decreases with increasing the concentration, while the salt rejection increases. When mineral oil is used for the organic phase, there exist certain conditions at which the formation of TFC membranes failed. It is most likely because high viscosity substantially reduces the diffusion rate of TMC, inhibiting TMC monomers from reacting with MPD monomers during interfacial polymerization.