Based on the consideration that the effective Hamiltonian matrix determining the zero field splitting parameter a of S-state ion in cubic crystal field possesses the same property with that of spin Hamiltonian matrix, a simple relation between parameter aand the non-diagonal elemente C of the effective Hamiltonian matrix in L-S representa-tion was derived as a=2/51/2C.As an example, the value of a of Mn2+ ion in CaO crystalwas calculated by using this relation, and the result agrees well with the experimental value.