The　nominal　short-circuit　current　breaking　capability　of　a　vacuum　circuit　breaker　(VCB)　is　closely　correlated　with　the　vacuum　arc　controlling　technology　and　the　opening　velocity　controlling　technology.　The　goal　of　this　paper　is　to　determine　an　opening　velocity　for　vacuum　interrupters　(VIs),　with　an　application　of　axial　magnetic　field　(AMF)　contacts,　based　on　the　evolving　of　vacuum　arc　intense　arc　mode　into　diffuse　arc　mode.　The　influences　of　the　opening　velocity　on　the　duration　of　the　intense　arc　mode,　the　critical　axial　magnetic　flux　density,　and　the　corresponding　critical　contact　gap　for　the　evolving　from　intense　arc　mode　into　diffuse　arc　mode　were　experimentally　determined　by　applying　different　opening　velocities　and　arcing　time　during　the　arcing　process.　Experimental　results　show　that　both　the　opening　velocity　and　the　arcing　time　have　a　significant　influence　on　the　evolvement　of　vacuum　arc　anode　discharge　mode.　The　higher　the　opening　velocity,　the　easier　for　the　intense　arc　mode　evolving　into　the　diffuse　arc　mode.　The　critical　contact　gap,　the　critical　magnetic　flux　density,　and　the　duration　for　the　intense　arc　mode　are　all　increasing　with　the　increasing　of　the　arcing　current,　during　the　evolvement　of　vacuum　arc　anode　mode　from　intense　arc　mode　into　diffuse　arc　mode.　The　dependence　of　the　critical　contact　gap　on　the　opening　velocity　shows　that　there　exists　a　peak　critical　contact　gap,　which　corresponds　to　a　threshold　opening　velocity.　For　the　design　of　the　opening　velocity　of　the　VCB,　the　preferred　velocity　should　be　the　higher　beyond　the　threshold　opening　velocity　the　better　for　the　intense　arc　mode　evolving　into　diffuse　arc　mode.　The　results　of　this　paper　theoretically　contribute　to　the　design　of　opening　velocity　of　the　VCB.　©　2018　Chin.　Soc.　for　Elec.　Eng.