A semiconductor/dielectric interface is one of the dominant factors in device characteristics, and a variety of oxides with high dielectric constants and low interface trap densities have been used in carbon nanotube transistors. Given the crystal structure of nanotubes with no dangling bonds, there remains room to investigate unconventional dielectric materials. Here, we fabricate carbon nanotube transistors with boron nitride nanotubes as interfacial layers between channels and gate dielectrics, where a single semiconducting nanotube is used to focus on switching behaviors at the subthreshold regime. The subthreshold swing of 68 mV·dec⁻¹ is obtained despite a 100-nm-thick SiO₂ dielectric, corresponding to the effective interface trap density of 5.2 × 10¹¹ cm⁻²·eV⁻¹_, one order of magnitude lower than those of carbon nanotube devices without boron nitride passivation. The interfacial layers also result in the mild suppression of threshold voltage variation and hysteresis. We achieve Ohmic contacts through the selective etching of boron nitride nanotubes with XeF₂ gas, overcoming the trade-off imposed by wrapping the inner nanotubes. Negligible impacts of fluorinating carbon nanotubes on device performances are also confirmed as long as the etching is applied exclusively at source/drain regions. Our results represent an important step toward nanoelectronics that exploit the advantage of one-dimensional van der Waals heterostructures.