The paper presents the results of determination of mechanical properties of seeds of di- and tetraploid red clover cultivars which were cultivated at different sowing density and in varied climatic conditions of the three years 2002-2004. The batches of seeds collected in each year were subjected to the test of uniaxial compression up to the anticipated seed deformation (75%) with the use of the universal test apparatus INSTRON. The measurements included the force of seed destruction related to the limit of biological strength, the energy (work) causing the destruction of seed structure, and a parameter of deformation which is the ratio between the increase of the force (F) and relative deformation (D). Statistical analysis showed significant contrasts between di- and tetraploid cultivars, sowing densities and years of study. All the measured characteristics were determined by the climatic conditions in the vegetation seasons. The cultivation of red clover under conditions of excessive precipitation caused an increase in all of the measured mechanical properties. The strength parameters of red clover seeds depended on the ploidity of cultivars which, depending on the climatic conditions, reacted variously. Besides, sowing density may also have had a significant impact on the seed mechanical parameters. It was observed that Fmax, Emax, as well F/D gave similar information on red clover seed strength.