pK(a) values of linear and cyclic methacrylic acid oligomers with 14 of degree of polymerization were measured in aqueous solution by pH titration and compared with that of poly(methacrylic acid), PMAA, prepared by radical polymerization. pK(a) of cyclic methacrylic acid oligomer, 4.83, was between those of linear methacrylic acid oligomer and PMAA, 4.69 and 5.05, respectively, suggesting that the hydrophobic field was formed with methyl groups in cyclic methacrylic acid oligomer in aqueous solution. In order to investigate the inclusion ability of cyclic methacrylic acid oligomer in aqueous solution, methylene blue was added to the solution of cyclic methacrylic acid oligomer. Interaction between methylene blue and cyclic methacrylic acid oligomer was investigated by UV-vis spectrometry at pH = 7.2 and 2D-NMR with nuclear Overhauser enhancement spectroscopy. Cyclic methacrylic acid oligomer enhanced the dimer formation of methylene blue. The size of hydrophobic field in cyclic methacrylic acid oligomer was estimated by MM2.