Standard enthalpies of formation ( $$\Updelta_f H^{0}$$ ) of finite-length (5, 5) single-walled carbon nanotubes (SWNTs) are calculated with the framework of density functional theory. Approximate expressions of ( $$\Updelta_f H^{0}$$ ) have been proposed for both H-terminated and C30-capped (5, 5) SWNTs, based upon which the calculated values of ( $$\Updelta_f H^{0}$$ ) have been reproduced within several kilocalories per mole. It is also found that standard enthalpies of formation contributed by per carbon, $$\Updelta_f H^{0}({\mathbf C}$$ ), oscillate with the increment of the cluster size, suggesting the dependence of the relative stability on the axial length.