Background:　Unbaffled　U-shaped　mesoscale　oscillatory　flow　reactors　(meso-OFRs)　are　able　to　achieve　low　pressure　drop,　good　plug　flow　and　micromixing　performance　simultaneously　at　laminar　net　flow　conditions　under　a　wide　＂operating　windows＂.Method:　We　focus　on　the　macromixing　and　micromixing　behaviours　of　unbaffled　meso-OFR,　where　the　net　flow　and　oscillation　conditions　were　in　the　wide　range　of　Ren=17-51　and　Reo=16-1350,　respectively.Significant　findings:　The　good　plug　flow　and　micromixing　performance　could　be　achieved　simultaneously　at　low　oscillation　intensity　with　the　wide　＂operating　windows＂　of　psi＜47.　It　was　believed　that　oscillatory　motion　led　to　periodic　destruction　and　reformation　of　vortices,　as　the　oscillation　intensity　increased,　the　large-scale　vortices　were　decomposed　into　small-scale　vortices,　resulting　in　backmixing　or　axial　dispersion.　Meanwhile,　the　empirical　formula　of　micromixing　time　and　velocity　ratio　in　accordance　with　the　power-law　relationship　was　established:　tm/t　=　0.00277　psi　0.51658.　Finally,　the　oscillatory　mechanism　was　qualitatively　validated　by　Computational　Fluid　Dynamics　(CFD).