We obtain defining equations of modular curves X 0 (N), X 1 (N), and X(N) by explicitly constructing modular functions using generalized Dedekind eta functions. As applications, we describe a method of obtaining a basis for the space of cusp forms of weight 2 on a congruence subgroup. We also use our model of X 0 (37) to find explicit modular parameterization of rational elliptic curves of conductor 37.