This paper solves a utility maximization problem under utility-based shortfall risk constraint, by proposing an approach using Lagrange multiplier and convex duality. Under mild conditions on the asymptotic elasticity of the utility function and the loss function, we find an optimal wealth process for the constrained problem and characterize the bi-dual relation between the respective value functions of the constrained problem and its dual. This approach applies to both complete and incomplete markets. Moreover, the extension to more complicated cases is illustrated by solving the problem with a consumption process added. Finally, we give an example of utility and loss functions in the Black-Scholes market where the solutions have explicit forms.If the price process S is locally bounded, then it is a local martingale under any equivalent local martingale measure Q on [0, T ]. Moreover, we denote by D(Q) the set of all Radon-Nikodym derivatives dQ/dP for any probability measure Q ∈ Q with respect to P .Assumption 2.2 We assume throughout the paper that Q = ∅.