In this work, the application of ionic liquids in separation of water from crude oil as a water demulsifier was reported. Imidazolium ionic liquids substituted with long alkyl chains (n=10, 12 and 14) were synthesized and used as surface-active demulsifiers which were coupled with Cl, PF6 and NTf2 anions. A full factorial design was performed for optimizing the behavior of demulsifiers based on three factors, i.e. alkyl chain length of cation, anion type and IL dosage in three levels. The measurements of interfacial tension between crude oil and water were done for each run and, the bottle test was carried out after demulsification process within the time intervals of 1, 2, 6 and 24 h. The optimal results for water separation efficiency and interfacial tension (IFT) were obtained 86-95% and 0.7-6.26 mN/m, respectively. The IFT reduction results showed that the studied ILs can be good candidates in enhanced oil recovery. It was found that increasing the ionic liquid hydrophobicity could significantly improve the demulsification efficiency along with IFT reduction. In hydrophobic ILs, it was indicated that increasing the alkyl chain length might improve the results, thus [C(14)mim][NTf2] showed the best results in the bottle test as well as IFT reduction to 0.8 mN/m with 2000 ppm. The changes in droplet size were monitored by optical microscopy.