Field experiments were conducted in oil and edible sunflower to study the effects of potassium (K) fertilization on achene yield and quality, and to estimate the nutrient internal efficiency (IE) and nutrient requirement in sunflower production. All trials in edible sunflower and 75% trials in oil sunflower showed positive yield responses to K fertilization. Compared with control without K fertilization, the application of K increased achene yield by an average of 406 kg ha–1 for oil sunflower and 294 kg ha–1 for edible sunflower. K application also increased 1 000-achene weight and kernel rate of both oil and edible sunflower. K fertilization improved the contents of oil, oleic acid, linoleic acid and linolenic acid in achenes of oil sunflower, and increased contents of oil, total unsaturated fatty acid and protein in achenes of edible sunflower. The average agronomic efficiency of K fertilizer was 4.0 for oil sunflower and 3.0 kg achene kg–1 K2O for edible sunflower. The average IE of N, P and K under balanced NPK fertilization was 22.9, 82.8, and 9.9 kg kg–1 for oil sunflower, and 27.3, 138.9, and 14.3 kg kg–1 for edible sunflower. These values were equivalent to 45.5, 14.1, and 108.1 kg, and 39.0, 8.0, and 71.7 kg of N, P and K, respectively, in above-ground dry matter required for production per ton of achenes. The average harvest index of N, P and K was 0.47, 0.56 and 0.05 kg kg–1 in oil sunflower, and 0.58, 0.58 and 0.14 kg kg–1 in edible sunflower.