In this work a phenomenological method is developed for the determination of spin configurations of magnetic crystals with a magnetic unit cell un-identical with its chemical unit cell. A finite group, formed of those elements of the space group which are distributed within a magnetic unit cell, is introduced. The expansion of the thermo-dynamic potential in terms of sublattice spin densities is constructed according to the principle that it should be invariant when symmetry operations of the above mentioned group are applied on sublattice spin densities separately. Spin configurations of magnetic crystals are then determined from the minimum condition of thermodynamic potential.With the above method, we have discussed the spin configurations of magnetic crystals of four different types. In each case, the results obtained are consistent with those of neutron diffraction experiments. Under certain conditions the existence of helical spin configuration may also be accounted for with the aid of this method.