Knowledge of mechanical properties of agricultural materials is important for designing and exploitation of agricultural machines and equipment, as well as for successful preservation of biological values of seed during drying, processing and storage. As the shape of soybean seeds is generally round, they can be graded by a simple procedure, using different sieve settings. In this study we assessed the impact of seed grade on the mechanical properties of seed of the soybean varieties which differed in seed size. The initial tangent modulus of elasticity and maximum rupture force were used as indices of mechanical properties of seed, and they were discussed in detail in the paper. The values of the initial tangent modulus of elasticity went down as seed size increased. The lowest value, 9.26 N/mm2, was recorded for grade 6.0 to 6.5 mm in the variety Balkan. The highest value, 18.84 N/mm2, was recorded for grade 4.0 to 4.5 mm in the variety Victoria. Unlike the values of the initial tangent modulus of elasticity, the values of maximum rupture force increased in proportion with increase in seed size. The highest value, 176.18 N, was recorded for grade 5.5 to 6.0 mm in the variety Victoria, and the lowest value, 92.15 N, was recorded for grade 4.0 to 4.5 mm in the variety Balkan.