To search for a novel sensor to detect the presence of formaldehyde (HCOH), we investigate reactivities of the intrinsic and boron-doped (B-doped) single-walled (8, 0) carbon nanotube (SWCNT) with HCOH using density functional theory calculations. Compared with the intrinsic SWCNT, the B-doped SWCNT presents high sensitivity to HCOH. This is attributed to the strongly chemical interaction between the electron-rich oxygen atom of HCOH and the electron-scarce boron atom of the doped SWCNT. B-doped SWCNTs are expected to be a potential candidate for detecting the presence of HCOH.