The behavior of alkyl guest radicals inside carbon nanotube hosts with different diameters is analyzed using density functional theory (DFT) calculations. Here the inner alkyl radicals are assumed to be formed by decomposition of their precursors, which had been incorporated into the tubes. DFT calculations show that inner alkyl radicals prefer to exist separately from the nanotube wall (separate form) rather than forming an inner covalent bond with the wall (bound form). Keeping a radical apart from the inner wall is more likely for a more bulky radical inside a smaller diameter tube. A key to the preference for the separate forms over the bound forms is that the bound forms gain a weak attraction due to the formation of a bond with the inner wall. The weak attraction, ascribed to the inertness of the inner surface, is counteracted by destabilization due to deformations of a tube and radical induced by guest-host coupling. The energy balance argument illuminates that the inertness of the inner wall makes an alkyl radical species remain alive inside a tube and retain its reactivity. These findings can help us to understand experimental results where chemical reactions inside a tube proceed after guests are activated.