Abstract: In the experiment，no significant difference in the seed size of watermelon inbred line BW85 with larger seeds and its small seed ,mutant F211 was observed within 7–11 days after pollination（DAP），11 DAP，the seed length，seed width，100-seed fresh and dry weight were 7.93 mm，4.53 mm，4.23 g，0.32 g and 8.26 mm，4.95 mm，4.54 g，0.30 g，respectively；but from 13 DAP，a significant difference in seed size was detected，the seed length，seed width，100-seed fresh and dry weight were 9.99 mm，6.12 mm，9.47 g，1.33 g and 8.86 mm，5.32 mm，4.60 g，0.46 g，respectively. Inbred line BW85 and mutant F211 were used to determine any differences of the testa development using paraffin section method. The results showed that the epidermis，hypodermis and sclerenchyma thickness of mutant F211 seeds（144.57 μm，130.53 μm，38.22 μm）were significantly smaller than those of inbred line BW85 seeds after 15 DAP. Furthermore，genetic analysis showed that all the plants in F1 and BC1P2 populations had small seeds，while segregation ratios of 3︰1 and 1︰1 was observed in F2 and BC1P1 proportions. The ratio was consistent with classical Mendelian law of inheritance，indicating that production of small seeds was dominant，and was controlled by a single gene.

Key words: watermelon, seed size, mutant, phenotype, inheritance