Magnetic susceptibility and magnetic excitation of antiferromagnetic and ferromagnetic (AF-F) alternating chains are studied by the pair dynamical correlated-effective-field approximation. The static magnetic susceptibility (chi)(T) vanishes at T = 0 K reflecting the existence of a gap between the singlet ground state and the excited states. At finite temperatures chi(T) shows a broad maximum around the temperature corresponding to the exchange coupling. The calculated chi(T) agrees fairly well with the results of exact diagonalization method for finite spins. The magnetic excitation exhibits a minimum value at q = +/-pi/2. This makes a striking contrast with the AF alternating chains in which the magnetic excitation has a minimum at q = 0 and q = pi. The magnetic susceptibility of real S = 1/2 organic AF-F chains, Cu(TIM)CuCl4 and [Cu(bpym) (OH)(2)].2H(2)O, are analyzed by our method and inelastic neutron scattering spectra for these systems are calculated.