In the paper a new, fractional order, discrete transfer function model of an elementary inertial plant is proposed. The model uses Atangana-Baleanu and discrete Fractional Order Backward Difference operators to describe of the fractional derivative. Such a transfer models have not be presented yet. The analytical formula of the step response for time-continuous transfer function is given. The similarity of the proposed model to “classic” one using Caputo operator is also considered. The stability and the convergence of the discrete transfer function are analyzed. Theoretical results are expanded by simulations. The proposed discrete, approximated model is accurate and its numerical complexity is low. It can be useful in modeling of different physical phenomena, for example thermal processes.