We have developed a new technique for the microwave-assisted purification of multiwalled carbon nanotubes (MWNTs) in a closed vessel at 180°C for 30 min in the presence of 20% hydrogen peroxide. A comparison of the purification efficiencies of hydrogen peroxide and nitric acid was conducted using the gravimetric method and indicated that the former exhibits better at removing defects than the latter. A temperature of 180°C was suitable for the purification of nanotubes without decomposition, as evidenced by Raman spectroscopy and thermogravimetric analysis. Purification temperatures below 180°C were not effective in removing defects, while temperatures above 180°C decomposed the nanotubes. SEM images provide further evidence for the effectiveness of purification at 180°C. This technique is more efficient, with regard to energy and time, than conventional purification methods.