Many studies on the plastic spin have been done, but it is still an opened subject. It is essential for the modelling of plastic spin to understand its origin and governing parameters. For the task, it is helpful to know the hehavior of sigle crystal. This study gives the interpretations for the plastic spin of a B.C.C(body centered cubic crystal) by numerical simulation. It is discussed that the different levels of shear stresses induced on each slip pair in crystallographic slip systems and the nonlinear relationship between shear stress and slip velocity are the fundamental mechanisms of plastic spins. Finally some comments are provided on the plastic spin of polycrystal.