Seed hardness is an important factor in determining soybean suitability for natto production. However, hardness testing is time consuming and labor intensive. The objectives for this study were to develop an efficient method for soybean hardness evaluation by examining different instruments and to investigate the relationship between the texture of steamed seed and other physical properties including seed size, swell ratio, and dimension change of soaked and cooked seeds. Five food-grade soybean lines (V97-6490, MFL-552, Hutcheson, MFS-591, and Camp) varying in seed size were analyzed with five different methods. All tests were conducted by one-bite method using two food-texture analyzers: TMS-2000 equipped with a shear cell and a TA-XT2i equipped with a single blade, a 2-mm probe, a 75-mm cylinder, or 16-probe pea rigs. The results showed that pea rigs using 16 seeds with five replications had the least coefficient of variation for seed texture among five testing methods. Shear cell was also a reliable method giving rise to highly reproducible results. Seed hardness was negatively correlated with seed size, swell ratio, and seed dimension change with soaking.