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Standard enthalpies of formation (Delta(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 (Delta(f)H(0)) have been proposed for both H-terminated and C30- capped ( 5, 5) SWNTs, based upon which the calculated values of (Delta(f)H(0)) have been reproduced within several kilocalories per mole. It is also found that standard enthalpies of formation contributed by per carbon, Delta(f)H(0) (C), oscillate with the increment of the cluster size, suggesting the dependence of the relative stability on the axial length.