Seed development in water convolvulus (Ipomoea aquatica Forsk.) was studied. Pod and seed development, at 5 days interval from 5-60 days after anthesis, was studied for physical changes such as size and color of pod, moisture content, germination, vigor and dry weight of seed. The result showed that pod size increased in diameter faster than in length and it reached maximum size at 30 days after anthesis while the color of pod was white. After fertilization, the seed moisture content decreased gradually. At 5-25 days after bloom, the seed increased its fresh and dry matter while the moisture content decreased. At 35 days after bloom, the moisture percentage was 51.37, after that the moisture decreased rapidly and was 14.72 percent at 40 days after bloom. Water convolvulus wwas physiologically mature at 35 days after bloom as determined by maximum dry weight. The seeds were capable to germinate at 20 days after bloom and then gradually increased to a maximum germination at the same time as maximum dry weight was attained. Seed hardness developed at 40 days after bloom and the moisture percentage was 14.72. Occurrence of hard seed was associated with seed moisture content. The decrease of the moisture content resulted in the increase of the number of hard seeds.