Thermo-responsive ionic liquids (ILs) were examined as draw solutions in forward osmosis (FO) membrane process. We synthesized several ILs to understand the relationship between the hydrophilicity/hydrophobicity balance of ILs and the osmolality and phase diagram of the IL aqueous solutions. An increase in the hydrophobicity led to a decrease in the osmolality because of the formation of IL-based aggregates in water. M the critical aggregation concentration (CAC), the osmolality was drastically changed. Above the CAC, the water-rich phase appeared after liquid-liquid separation at elevated temperature. The osmolality, phase separation temperature, viscosity, and water-drawing ability of the IL-based draw solution could be controlled by controlling the chemical structure and hydrophilic/hydrophobic balance of ILs. The IL with small molecular weight and low viscosity showed the comparable water permeability as the NaCl aqueous solution and the possibility of repeated use. Prevention of the IL leakage through FO membrane is a future subject.