The calculation method for transition state of oscillator strength suggested by Ellis and Gosoinski [Physica Scripta, 9, 104 (1974)] has been modified for considering spin polarization. By means of this version, f-values for s-p, p-s and p-d transitions between the ground and or lower excited states in atom Li are obtained. It is further shown that the data obtained by using this method approach the tabulated values better than ones by using some more complex methods such as Hartree-Fook method for some transitions. This may reveal the physical meaning of the transiiion state in calculation of osuillator strength.