We report a size-exclusion chromatography (SEC) process to purify DNA-wrapped carbon nanotubes (DNA-CNT) and to sort them into fractions of uniform length. A type of silica-based column resin was identified that shows minimum adsorption of DNA-CNT. Three such columns in series with pore sizes of 2000, 1000, and 300 A were found to separate DNA-CNT into fractions of very narrow length distribution, as measured directly by atomic force microscopy. The average length decreases monotonically from > 500 nm in the early fractions to < 100 nm in the late fractions, with length variation < or = 10% in each of the measured fractions. Using UV-vis-NIR spectroscopy, we showed that SEC is very effective in removing graphitic impurities that contribute to the spectral baseline and a broad absorption peak at approximately 270 nm. This result highlights the importance of CNT purification in the study of optical properties of CNT.