Background:　Unbaffled　U-shaped　mesoscale　oscillatory　flow　reactors　(mesoOFRs)　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　mesoOFR,　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　ψm/t¯=0.00277ψ0.51658.　Finally,　the　oscillatory　mechanism　was　qualitatively　validated　by　Computational　Fluid　Dynamics　(CFD).　©　2023