Understanding　the　competing　modes　of　brittle　versus　ductile　fracture　is　critical　for　preventing　the　failure　of　body-centered　cubic　(BCC)　refractory　metals.　Despite　decades　of　intensive　investigations,　the　nanoscale　fracture　processes　and　associated　atomistic　mechanisms　in　BCC　metals　remain　elusive　due　to　insufficient　atomic-scale　experimental　evidence.　Here,　we　perform　in　situ　atomic-resolution　observations　of　nanoscale　fracture　in　single　crystals　of　BCC　Mo.　The　crack　growth　process　involves　the　nucleation,　motion,　and　interaction　of　dislocations　on　multiple　1/2　＜　111　＞　{110}　slip　systems　at　the　crack　tip.　These　dislocation　activities　give　rise　to　an　alternating　sequence　of　crack-tip　plastic　shearing,　resulting　in　crack　blunting,　and　local　separation　normal　to　the　crack　plane,　leading　to　crack　extension　and　sharpening.　Atomistic　simulations　reveal　the　effects　of　temperature　and　strain　rate　on　these　alternating　processes　of　crack　growth,　providing　insights　into　the　dislocation-mediated　mechanisms　of　the　ductile　to　brittle　transition　in　BCC　refractory　metals.