Single-walled carbon nanotubes were dispersed by tip-type sonication for 20, 120 and 180 min in the presence of surfactant. The diameter distributions of the three dispersions were compared by absorption spectra, indicating that absorbance of the larger diameters relatively decreased as the time of tip-sonication became longer. That is, the amount of larger diameters gradually decreased as compared with that of smaller ones according to the sonication time.