The fabrication of p-n heterostructures of a metal oxide semiconductor (MOS) showed that a large amount of heterojunction interfaces is one of the key issues in MOS gas sensor research, since it could significantly enhance the sensing performance. Despite considerable progress in this area, fabrication of an ideal p-n heterojunction sensing channel has been challenging because of morphological limitations of synthetic methods in the conventional bottom-up fabrication based on precursor reductions. In this study, a 10 nm scale p-n heterojunction nanochannel was fabricated with ultrasmall grained WO3/CuO nano-patterns in a large area (centimeter scale) through unique one-step top-down lithographic approaches. The fabricated p-n heterostructure nanochannel showed ultrathinness (20 nm thickness) and high aspect ratio (>10) and consisted of highly dispersed p-type dopants and n-type channel materials. This facile heterojunction nanostructure could induce a high degree of extended depletion layer and efficient catalytic properties within its single-nanochannel surfaces. Accordingly, the WO3/CuO nanochannel exhibited ultrasensitive detection performance toward ethanol (C2H5OH) (R-a/R-g = 224 at 100 ppb), 12 times higher than that of a pristine WO3 nanochannel. The limit of detection of the sensors was calculated to be below parts per billion levels (0.094 ppb) with significant response amplitudes (R-a/R-g = 75), which is the best ethanol-sensing performance among previously reported MOS-based sensors. Our unique lithographic approach for the p-n heterojunction nanochannel is expected to be universally applicable to various heteronanostructures such as the n-n junction, p-p junction, and metal-semiconductor junction without combinatorial limitations.