Lattice structures are found in many natural systems at the microscopic level and have also been adopted in engineering at many scales due to their high strength-to-weight ratio and energy absorption properties. The assessment of load-bearing capacity for their safe utilization is crucial. Moreover, such structures are prone to defects, both from the geometrical and the mechanical side and so this has to be taken into account for a proper mechanical characterization. However, a parametric study on imperfections requires to test a lot of cases, and non linear incremental micro-mechanical analysis for such structures are cumbersome due to the number of elements. Then, since it has been shown that limit analysis is capable of predicting the strength of these structures, an upper bound approach has been adopted for the purpose. Mechanical imperfections are generated, and a Monte Carlo simulation is employed to determine the influence of defects on the strength of lattice structures. The mechanical defects are considered both randomly distributed and localized. Results show that defects have an influence on the behaviour of lattice structures which is dependent on the lattice geometry. Moreover defects di not only influence the strength but they also affect the failure mode.