Abstract: The [2_s+n_a] system can have C_2 symmetry (Fig.1) rather than local C_2 and local σ as described in Ref.4. For the [m_s +n_a] cycloaddition with m>2, the reaction system is no doubt unsymmetrical, the special symmetry of the MOs obtained by the HMO model is due to oversimplification. Bond angles are not concerned in the HMO method. Thus, if the m_s component is regarded as a straight carbon skeleton, the system will be of C_2 symmetry, which is shown in Fig.1 with the m_s in place of the ethylene. Correlation diagrams of [m_s+n_a] canbe constructed from the MO energies and special symmetries of the initial and the final states. For [m_s+m_a] reaction, the eigenvalue problem is equivalent to that of two identical systems with straight carbon chains (Fig.3). Therefore, double degeneracy holds for all MOs throughout the reaction (Fig.4).