Thermal properties are studied for a spin system described by a Hamiltonian which contains an Ising term, a Heisenberg term and a coupled Ising and Heisenberg interaction term. This system has three order parameters. By using the mean-field approximation, thermal variations of the order parameters are studied for positive values of the interaction constants. Landau theory is applied to determined the critical temperatures. Phase boundaries of the system are given. It is also shown that the mean-field theory is exact when the range of interaction is infmite.