Modifying the spin-wave theory in the three-dimensional Heisenberg ferromagnet, we propose a simple approximation to investigate the low-temperature thermodynamics of one- and two-dimensional Heisenberg ferromagnets H = -J Σ≪ij> (SiSj - S2), where S is the spin quantum number. We regard that the system is an ideal Bose gas with density S. For one-dimensional chain we get the free energy and the susceptibility per site: