Mechanical strength of a nanotube bundle has been measured by using quartz glass enclosed-purified SWNTs as the specimen. These specimens were prepared by fast stretching quartz glass tube that contained purified nanotube bundles, at temperatures ranging from 900 to 1490°C. The maximum average critical load was 42.3 kg/mm2 for the specimen prepared at 900°C. However, at temperatures higher than 900°C, the critical load value decreased and became almost equal to that of quartz glass at 1490°C. It was believed that the decrease in the critical load value at temperatures higher than 900°C was due to the partial conversion of nanotubes into amorphous carbon. Since the cross-sectional area used in arriving at the critical load value was that of nanotube and the quartz glass, the cross-sectional area ratio of the quartz to that of the nanotube suggested that the mechanical strength of nanotube would be two orders of magnitude higher than the value reported here.