The　critical　condition　for　bubble　breakup　in　a　microfluidic　flow-focusing　junction　was　studied　in　this　work.　The　experiments　were　conducted　in　a　square　microchannel　of　400　mu　m　wide.　The　critical　condition　for　bubble　breakup　was　investigated　by　varying　the　bubble　length　l(0),　liquid　viscosity　mu,　velocity　of　the　liquid　from　the　main　channel　of　the　flow-focusing　junction　u(1),　and　velocity　of　the　liquid　from　the　side　channels　of　the　junction　u(2).　By　analyzing　the　effects　of　these　factors　on　the　dynamical　evolution　of　gas-liquid　interface　for　bubble　deformation　and　breakup,　expressions　for　describing　the　bubble　deformation　and　breakup　time　were　established,　respectively.　On　the　basis　of　these　two　expressions,　the　critical　condition　for　the　bubble　breakup　in　a　microfluidic　flow-focusing　junction　was　deduced:　l(0/)w(c)　=　1.5(u(1)/u(2))(0.75)　ca(2)(-0.13)　,　where　w(c),　is　the　width　of　microchannel,　Ca-2　=　u(2)mu/gamma,　and　gamma　is　the　surface　tension.　(C)　2017　Elsevier　Ltd.　All　rights　reserved.