We propose a phenomenological model to describe the tricritical behavior of the nematic to smectic- A (N-SmA) phase transition in liquid crystal mixture. To describe the mesophase transitions in binary mixture, nematic and smectic order parameters have been coupled with the concentration. We show that a tricritical point on the N-SmA phase transition line can be achieved under certain conditions. The predictive capability of the present model for determining the tricritical point of a binary mixture displaying the N-SmA transition has been demonstrated by testing with reported phase diagrams sharing both phases