Metamagnetism observed in some heavy fermion compounds is an intriguing phenomena. To understand this phenomena, we explore the phenomenological mean-field theory for heavy fermion systems without microscopic mechanism. We write an extension of the Landau-like free energy density with the Bragg-Williams entropy. By minimising this free energy density with respect to the sub-lattice magnetizations, we can obtain thermal equilibrium quantities. As examples, the magnetic behaviors of CeRu_2Si_2 (the metamagnetism with crossover) and CeTiGe (the first order metamagnetism) are explained qualitatively, where the scaling hypothesis is used to consult with experimental data under the constant pressure condition.
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