Polymerization of phenol with paraformaldehyde and catalytic sulfuric acid was carried out in room temperature ionic liquids based on imidazolium salts. The properties of ionic liquids were systematically varied by employing six different anions such as chloride, bromide, iodide, tetrafluoroborate, trifluoromethanesulfonate and hexafluorophosphate. In case of hydrophilic ionic liquids having chloride or bromide anion, phenol polymers with low molecular weight and narrow polydispersity were obtained. In contrast, using hydrophobic ionic liquids such as tetrafluoroborate and trifluoromethanesulforate anions, high molecular weight phenol polymers were prepared. It is because interaction of the hydrophilic ionic liquids with phenol is stronger than that of the hydrophobic ionic liquids. The interaction between phenol and ionic liquids was investigated by H-1 NMR measurements. Moreover, by using Bronsted acid ionic liquid carrying sulfuric acid moiety as a solvent, catalytic acid-free green polymerization of phenol with paraformaldehyde was accomplished. The ionic liquid was efficiently recovered and reused in new polymerization process of phenol.