In this work we studied the thermal stability of single walled MoO3 nanotubes using Raman spectroscopy technique. By increasing the thermal treatment temperature we observed at about 210°C that the Raman spectrum characteristic of single walled MoO3 nanotubes changes and the new spectrum is identified to the MoO3 bulk orthorhombic phase (α-MoO3). In situ Raman measurements as a function of temperature increase confirm the collapse of the nanotube structure at about 210°C. Scanning electron microscopy images confirm the morphology changes in the single walled MoO3 nanotubes for samples treated at different temperatures.