By electron beam evaporation, noble metals (Au, Co, Pt, and Ag) with 2 and 4 nm nominal thicknesses were coated onto multi-walled carbon nanotube layers. The metals were in the form of nanoparticles mounted onto the side walls of carbon nanotubes (CNTs) to create a metal/CNT junction. The CNTs were directly grown on patterned Pt-electrode alumina substrates through chemical vapor deposition to produce a resistivity-based ammonia gas sensor. The metallic surface-modified CNT-based sensors were found sensitive to NH3 gas at room temperature. Compared with pristine CNT sensor, the response of Au/CNTs sensor increased slightly, whereas the responses of the Pt/CNTs, Co/CNTs, and Ag/CNTs increased by two, three, and more than four times, respectively.