A microscopic theoretical model is considered to explain the dynamic properties of magnetic system of Ho 0.4Y 2.6Fe 5O 12. The model hamiltonian contains the exchange interaction between the iron spins, the antiferromagnetic exchange interaction between the iron spins and the rare-earth Ising spins and the Zeeman energy. The experimentally observed anomalies of magnetic resonance are found to appear due to strong fluctuations of the rare-earth Ising spins and their maximal spin-lattice relaxation in the vicinity of magnetic phase transition.