The quasi-continuum method (QC), a multiscale method, is used to analyze body-centered-cubic (bcc) metal tantalum (tantalum, Ta) type Ⅱ crack-tip dislocation nucleation. Based on the relationship curves between dislocations emission position and stress intensity factor, the processes of dislocation defect initiation and development are investigated. Dislocation travels forward with different characteristics in different stages and the new nucleated dislocations expedite the already nucleated dislocation to move away from the crack tip. The analysis of initiation of the crack tip defects shows that they are the local defects that first appear, and with loading, more local defects emerge, which eventually move to the boundary, and lead to the type Ⅱ fracture. Furthermore, dislocation dissociation as well as extended dislocation is discussed. The partial dislocation nucleating before the perfect dislocation nucleation and emission is full proof that the dissociation of perfect dislocation takes place step by step, which means that the two minimum points on the energy curve have different formation mechanisms.