The effect of substituting Mn by Co and Cr on the magnetic properties of layered perovskite manganese oxide La1.2Sr1.8Mn2O7 is investigated. Co doping results in the frustration of long-range ferromagnetic (FM) order, weakening of ferromagnetism, and occurring of cluster-glass and spinglass state, indicating that no double-exchange interaction occurs between Co and Mn ions. However, the long-randge FM order still exists as Mn is replaced by Cr, which verifies that there is FM interaction between Mn3+ and Cr3+.