The slip behavior and mechanism of large-size Mo-3Nb single crystal have been investigated and dis-closed comprehensively at room temperature by quasi-static compression with various strains.With the increase of deformation,the slip traces change from shallow non-uniform slip lines to dense and uniform slip bands.Different slip traces in the same deformation condition were observed,suggesting that the slip traces in the single crystal are controlled by different types and arrangement mechanisms of mo-bile dislocation.To clarify the relationship between slip behavior and dislocation arrangement,TEM and AFM analyses were performed.Significant discrepancy between the mobility of screw segments and edge segments caused by double cross-slip multiplication mechanism is the reason why different slip features were witnessed.During the whole slip deformation process,screw dislocations play a dominant role and they are inclined to form wall-substructures by interaction and entanglement.With the development of dislocation accumulation,the entangled dislocation walls evolve into dislocation cells with higher stabil-ity.