Ordered two-dimensional (2D) arrays of beta-HgS nanocrystal aggregates were prepared successfully at the air/water interface through the interfacial reaction between Hg2+ ions in the subphase and H2S in the gaseous phase under the direction of liquid-expanded monolayers of arachidic acid (AA) These 2D arrays are composed of hexagonal or quasi-hexagonal aggregates with the size of several hundreds of nanometers that consist of several tens of HgS nanocrystals with the size of several nanometers The formed HgS nanocrystals together with AA molecules self-assembled into round aggregates due to the interactions between the species, and the aggregates self-assembled into 2D arrays further clue to the attractions between them During the self-assembly process, the soft round aggregates transformed into hexagonal or quasi-hexagonal ones The experimental conditions, especially the phase states of the AA monolayers and temperature, have great influences on the formation of the 2D arrays To the best of our knowledge, this is the first case to get 2D ordered arrays at the air/water interface through a one-step synthesis and assembly process