Time-resolved　dislocation　tangle　and　untangle　processes　in　small-sized　body-centered　cubic　(BCC)　niobium　single　crystal　samples　were　investigated　using　in-situ　nanoindentation　transmission　electron　microscopy.　The　results　reveal　that　dislocations　can　not　only　undergo　tangle　processes　but　can　also　untangle　during　plastic　deformation,　which　have　been　predicted　theoretically　but　not　observed　experimentally.　The　formation　of　dislocation　tangles　is　attributed　to　the　high　mobility　of　dislocations　with　a　high　density　of　sessile　defects　in　the　deformed　BCC　crystal,　whereas　the　untangle　process　arises　from　increased　stress.　The　strength　of　dislocation　tangles　in　our　experiments　was　calculated　based　on　widely　used　models,　which　showed　good　agreement　with　previous　simulations.　Dislocation　tangle-untangle　processes　lead　to　work-hardening　and　high　strength　in　small-sized　BCC　metals.　(C)　2017　Elsevier　B.V.　All　rights　reserved.