Additive technologies offer promising opportunities for creating structures with optimal properties tailored to the requirements of each particular application. The present paper is focused on functionally graded lattice structures, which are designed based on the triply periodic minimal surfaces (TPMS) of the Gyroid and I-WP types. The aim of the work was to create three-dimensional models and to study mechanical behavior of the polymer lattice structures that can be obtained by the fused filament fabrication (FFF) 3D printing method. The geometrical models of heterogeneous lattice structures were obtained using the level-set approach and equations for the periodic surfaces. The results of modeling of the mechanical behavior of lattice structures under uniaxial tension were obtained using the finite element analysis. Influence of structural parameters and gradient properties on the distribution of stress fields was studied by comparing loading curves obtained for the structures in the course of the numerical simulations.