Mylonchulus laocaiensis sp. n. recorded from Vietnam, is described and illustrated, and its phylogenetic relationship within the Mylonchulidae family and Mononchida order are analysed. The new species is characterized by medium body size (L = 1.0–1.5 mm); buccal cavity goblet shaped, 26–29 × 14–16.5 μm or 1.8 (1.7–1.9) times as long as wide; posterior position of dorsal tooth apex (79%–88% from the base of buccal cavity); a small subventral tooth present on subventral wall; pars refringens vaginae with faint and small (2.5 × 1.7 μm) teardrop-shaped pieces, short pars distalis vaginae; male with short spicules (54–57 μm) with rounded head. The molecular data (18S and 28S rDNA) are provided for the new species.