In this paper, we first give a sufficient condition on the coefficients of a class of infinite time interval backward stochastic differential equations (BSDEs) under which the infinite time interval BSDEs have a unique solution for any given square integrable terminal value, and then, using the infinite time interval BSDEs, we study the convergence of g-martingales introduced by Peng via a kind of BSDEs. Finally, we study the applications of g-expectations and g-martingales in both finance and economics.2000 Mathematics subject classification: primary 60H10, 60G48. Keywords and phrases: infinite time interval BSDEs, g-expectation, g-martingale, upcrossing inequality of g-martingale, convergence of g-martingale.The adapted solution for a linear BSDE which appears as the adjoint process for a stochastic control problem was first introduced by Bismut in 1973, then by Bensoussan and others, while the first result for the existence and uniqueness of an adapted solution to a nonlinear BSDE with finite time interval and Lipschitzian coefficient was obtained by Pardoux and Peng [20]. Later many researchers developed the theory and its applications in a series of papers (see for example Darling [5] Karoui, Peng and Quenez [8] and the references therein) under some other assumptions on coefficients but for fixed terminal time. From these papers, the basic theorem is that, for a fixed terminal time T > 0, under the suitable assumptions on terminal value £, coefficient g and driving process M, the following BSDE has a solution pair (y,, z,)