Presence of hard seeds in seed lots reduces the seedling emergence percentage in field leading to non-uniform and lowered plant population and uneven maturity. Seed hardness, characterized by no water imbibitions, is controlled by both genetic and environmental factors. The occurrence of hardseededness in 51 mung bean genotypes grown in the field in two seasons (summer and Kharif, 2016) was confirmed by standard germination test. The seed size, 100 seed mass and seed density of identified 20 hard seed and 8 non-hard seed genotypes out of 51 genotypes have been recorded. Hardseededness among the genotypes in original seed lot of Kharif, 2015 ranged from 3 to 52% with a mean of 18.1%. It ranged from 2% to 11% and 3% to 46%, with a mean of 5.42% and 18.35% in summer, 2016 and in kharif, 2016, respectively. In the seed lots of genotypes grown in kharif, 2015 the seed size of selected hard seed genotypes ranged from 3.69/2.90 mm (L/W) to 4.58/3.31 mm (L/W) and in selected non-hard seed genotypes it ranged from 4.01/2.46 mm (L/W) to 4.83/3.48 mm (L/W). The 100 seed mass in selected hard and non-hard seed genotypes ranged from 2.69 to 4.13 g and 2.39 to 3.94 g, respectively, in kharif, 2015. In summer, 2016, it ranged from 3.57 to 4.75 g and 3.05 to 5.58 g in selected hard seeded and non-hard seeded genotypes, respectively. A higher seed density of hardseeded genotypes was recorded in comparison to that of non-hard seeded genotypes. In summer, 2016 the genotypes like Pant M-5, GM-4, IPM 02/3, AKM 9904, PS-16, SM-11/75, HUM-12, IPM 205/7 showed lower occurrence of per cent hard seeds. The broad sense heritability estimates for the hardseededness in the kharif and summer season ranged between 0.96 to 0.98 and 0.67, respectively. A larger seed size in genotypes grown in summer season showing lower occurrence of hard seeds suggest that the summer season is the desired season for quality seed production of genotypes with higher percentage of hard seeds.