Circadian　clocks　drive　rhythmic　physiology　and　behavior　to　allow　adaption　to　daily　environmental　changes.　In　Drosophila,　the　small　ventral　lateral　neurons　(sLNvs)　are　primary　pacemakers　that　control　circadian　rhythms.　Circadian　changes　are　observed　in　the　dorsal　axonal　projections　of　the　sLNvs,　but　their　physiological　importance　and　the　underlying　mechanism　are　unclear.　Here,　we　identified　miR-263b　as　an　important　regulator　of　circadian　rhythms　and　structural　plasticity　of　sLNvs　in　Drosophila.　Depletion　of　miR-263b　(miR-263b(KO))　in　flies　dramatically　impaired　locomotor　rhythms　under　constant　darkness.　Indeed,　miR-263b　is　required　for　the　structural　plasticity　of　sLNvs.　miR-263b　regulates　circadian　rhythms　through　inhibition　of　expression　of　the　LIM-only　protein　Beadex　(Bx).　Consistently,　overexpression　of　Bx　or　loss-of-function　mutation　(Bx(hdpR26))　phenocopied　miR-263b(KO)　and　miR-263b　overexpression　in　behavior　and　molecular　characteristics.　In　addition,　mutating　the　miR-263b　binding　sites　in　the　Bx　3　＇　UTR　using　CRISPR/Cas9　recapitulated　the　circadian　phenotypes　of　miR-263b(KO)　flies.　Together,　these　results　establish　miR-263b　as　an　important　regulator　of　circadian　locomotor　behavior　and　structural　plasticity.