A new simple and accurate method for molecular weight estimation of oligomers by reversed-phase liquid chromatography was developed and has been proposed to use in the investigation of bio-oligomers. Capacity factors (k′) of 24 peptides and proteins ranging in molecular weight from 200 to 70000 were measured independently under isocratic eluting conditions with slightly different acetonitrile contents in the mobile phase. Profiles of plots of the log k' values of the peptides and proteins against the acetonitrile contents were on straight lines with different slopes. These slopes were characteristic for each oligomer and found to be proportional to the two/thirds power of the molecular weight. This elution behavior of oligomers in reversed-phase liquid chromatography can be explained in terms of the solvophobic theory. The estimated molecular weights of several bio-oligomers were more accurate than those obtained by size-exclusion chromatography. when the range of molecular weights was limited from 103to 2×104.