  The spin degrees of freedom for the relativistic particle are described by
either Poincar\'{e} group variables (classically) or Grassmann variables
(pseudo-classically). The relationship between those two descriptions are
given. In doing that, appropriate constraints are constructed to put into the
lagrangian. Especially a natural relation of Poincar\'{e} group variables and
Grassmann variables is obtained. Hopf fibration relating the spin momentum to
the group is just the right transformation of the spin momentum under
Poincar\'{e} group. And with the relation just mentioned, pseudoclassical
lagrangian is derived naturally from the classical one.
