We investigated the formation of nanowhiskers by means of air plasma dry etching using diamond films of two different kinds: as-grown diamond films and films with molybdenum (Mo) deposits. As for the as-grown diamond films, nanowhiskers were found to form preferentially at grain boundaries of diamond crystals. Auger depth profile analysis of the etched films revealed a progressive enrichment by Mo toward the whisker tip, resulting from accidental sputtering of Mo substrate holder. With dry etching of diamond films with preformed Mo deposits, well-aligned whiskers 100 nm in diameter were found to form uniformly over the entire film surface with a population density of 30/μm2. From these findings, it follows that Mo deposits serve as micromasks for the formation of the nanowhiskers. It was also confirmed that these whiskers showed excellent field-emission behavior.