A finite element optimization method is presented that determines the optimal thickness profile of a preform for a blow-molded part having the required wall thickness distribution. The optimization method is based on a method of feasible directions and the design variables are thicknesses of finite elements. The step size is determined by using a Brent 1-dimensional minimization method. Two methods for determining feasible directions are compared. A finite element model is formulated based on the thin membrane approximation, which relates the preform wall thickness distribution to the wall thickness distribution in the blow-molded part. Triangular or conical frustra elements are used to describe deformed shapes of the preform for a-dimensional axisymmetric deformations. An incompressible nonlinear elastic constitutive model describes the material response. The accuracy of the finite element model used is evaluated by comparing the model predictions to published experimental data and computer simulations.