Tight Binding molecular dynamics simulations have been performed on single wall carbon nanotubes, in order to evaluate thermal stability and the effect of the most relevant defects (the single vacancy and a Stone-Wales -SW- defect). The nanotubes are stable up to the graphite instability temperature. Both the considered defects have a large formation energy (EF(vac)=6.10 eV, EF(SW)= 5.55 eV).