Iran is one of the watermelon producer countries and in terms of watermelon cultivated area, is in second globally. Kerman Province is in the southeast of Iran which ranked first in watermelon producing provinces. Whereas in recent decades, drought, physical and chemical properties of soil, poor quality irrigation water, lack of proper and balanced nutrients have caused outbreak of blossom-end rot, quantitative and qualitative reduction of watermelon. Therefore, simultaneous application effect of organic matter, potassium and calcium on yield, chemical composition and blossom-end rot of watermelon was investigated that was new study on this crop. The variables were included organic matter (0 and 20 ton/ha of animal manure), potassium sulphate (0, 250 and 350 kg/ha) and calcium chelate foliar application (0 and 4 g/lit). The project was conducted with 12 treatments and three replications in Randomized Complete Block Design in one of counties in southeast of Iran. Results revealed that organic matter application and foliar calcium chelate caused increase in yield, calcium, iron and zinc concentration in the plant and decrease in blossom-end rot of watermelon fruit significantly. Moreover, potassium sulphate led to yield increase but application more than 250 kg/ha caused concentration reduction of calcium, iron and zinc in plant and increase in blossom-end rot.