Based on the calculation with a new parameter-"displacement vector" for martensitic transformation deduced by authors, the limitation of the phenomenological theory has been pointed out. Through the calculation for displacive grad tensor of the lattice strain during martensitic transformation, it has been found that the lattice deformation of the martensite includes a pure strain and a rigid rotation, which can be described by matrix quantitatively. Consequently, the largest pure strain and shear strain can be obtained as 13.85% and 17.41% (shear angle 9.88degrees) respectively in the case of FCC-->BCC martensitic transformation.