Facile　assembly　of　intelligent　DNA　nanoobjects　with　the　ability　to　exert　in　situ　visualization　of　intracellular　microRNAs　(miRNAs)　has　long　been　concerned　in　the　fields　of　DNA　nanotechnology　and　basic　medical　study.　Here,　we　present　a　driving　primer　(DP)-triggered　polymerization-mediated　metastable　assembly　(PMA)　strategy　to　prepare　a　well-ordered　metastable　DNA　nanoarchitecture　composed　of　only　two　hairpin　probes　(HAPs),　which　has　never　been　explored　by　assembly　methods.　Its　structural　features　and　functions　are　characterized　by　atomic　force　microscope　(AFM)　and　gel　electrophoresis.　Even　if　with　a　metastable　molecular　structure,　this　nanoarchitecture　is　relatively　stable　at　physiological　temperature.　The　assembly　strategy　can　be　expanded　to　execute　microRNA-21　(miRNA-21)　in　situ　imaging　inside　cancer　cells　by　labelling　one　of　the　HAPs　with　fluorophore　and　quencher.　Compared　with　the　conventional　fluorescence　probe-based　in　situ　hybridization　(FISH)　technique,　confocal　images　revealed　that　the　proposed　DNA　nanoassembly　can　not　only　achieve　greatly　enhanced　imaging　effect　within　cancer　cells,　but　also　reflect　the　miRNA-21　expression　level　sensitively.　We　believe　that　the　easily　constructed　DNA　nanoarchitecture　and　in　situ　profiling　strategy　are　significant　progresses　in　DNA　assembly　and　molecule　imaging　in　cells.　©　2016　Elsevier　Ltd