This experiment was conducted in South Tahreer Horticultural Research Station during the 1999 and 2000  seasons, with the aim of investigating the effect of different K fertilization levels (0, 50, 75 or 100 Kg K₂SO₄ fertilizer/fed.) and harvest timing (after 163, 170, 177, 184, 191 or 198 days from planting) on the growth, sepal and seed yields, as well as fixed oil content of roselle (Hibiscus sabdariffa L.) seeds. Potassium fertilization at 75 Kg K₂SO₄ fertilizer/fed. gave the best results in terms of the number of fruits/plant, fresh weight of fruits, fresh and dry weights of sepals, seed yield per plant and per feddan, seed index (weight of 100 seeds), seeds oil content (%), as well as the oil yield per plant and per feddan. Raising the K rate resulted in a steady increase in the K content of leaves, but reduced the anthocyanin content of sepals steadily. The tallest plants were those fertilized at 100 Kg K₂SO₄ fertilizer/fed. Moderate K fertilization (50 or 75 Kg K₂SO₄ fertilizer/fed.) gave the highest N content in the leaves. Delaying the harvest of fruits resulted in a steady increase in the seed yields per plant and per feddan, the N content (%) in leaves, and the anthocyanin content in sepals. All of these parameters gave their highest values after 198 days from planting. The highest oil yield (per plant and per feddan) was also obtained at the same harvest timing (198 days after planting). On the other hand, harvesting the fruits one week earlier (after 191 days from planting) gave the highest number of fruits/plant, as well as the highest values for fruits fresh weight, fresh and dry weights of sepals, and seed index. Maximum plant height was also recorded after 191 days from planting. The highest P and K contents in leaves were obtained after 163 and 184 days from planting, respectively, while the highest oil content in the seeds was recorded after 184 days from planting. From the above results, it can be recommended that roselle plants should be fertilized with 75 Kg K₂SO₄ fertilizer/fed., and should be harvested after 191-198 days from planting.