This research aims to understand the most effective nano potassium spraying concentration of the empty oil palm to increase mass and yield of the onion (Allium ascalonicum L). Research did in December 2018 and ended in February 2019 on the land of the Agriculture Faculty Universitas Muhammadiyah Yogyakarta. This research is written using experimental methods by the single designed factor consist of five treatments arranged in Complate Random Design (RAL). Treatment that was tasted between the KCL fertilizer concentration and the ash of empty palm oil bunch. The treatments are consisted of without KCl fertilizer; 100 % KCl + without the nano potassium of TKKS; 50% KCl + the nano potassium of TKKS 0,2%; 50% KCl + the nano potassium of TKKS 0,3%; 50% + the nano potassium of TKKS 0,4%. The result showed that giving the nano potassium of TKKS and KCl did not have a significant effect on all growth parameters, but gave a significant effect on the result parameters of fresh tuber weight, tuber dry weight and tuber yield (ton/ha) at week 8 after planting. Therefore, the application of nano potassium from TKKS can substitute the synthesis of fertilizer usage, such as KCl. Treatment in 50% KCl + 0,4 % nano potassium of TKKS effective gave increase the results of 18,66 tons/ha.