It is shown how the propagator formalism can be used to obtain the low‐temperature expansion of the free energy of an isotropic Heisenberg antiferromagnet. The lowest‐order terms in such an expansion can be calculated using the proper self‐energy evaluated at zero temperature. The analytic properties of this quantity are investigated by expressing it in terms of time ordered diagrams. The low‐temperature expansion of the free energy is shown to be of the form AT 4+BT 4+CT 8, where A, B, and C are given by Oguchi correctly to order 1/S. For spin ½ the term in 1/S 2 gives a 2% reduction in A for a body‐centered lattice.