We establish existence results of homoclinic orbits of the first order time-dependent Hamiltonian system (z)over dot = JH(z) (t, z), where H(t, z) depends periodically on t, H(t, z) = (1)/(2)zL(t)z + W(t , z), L(t) is a symmetric matrix valued function and W(t, z) satisfies certain global superquadratic condition. We relax partly the assumption often used before: L is independent of t and sp(JL)boolean AND iR = phi.