To investigate the effects of potassium($K_2O$) fertilization levels on the dry matter(DM) yield, nutrition yield, and $K_2O$ efficiency of forages in mixed sward of alfalfa-grasses, a field experiment arranged by randomized complete block design with five treatments(0, 60, 120, 180 and 240kg $K_2O$/ha) was conducted at National Livestock Research Institute in Suwon, September, 1990 to February, 1993. During two years, average DM yields of forage increased as $K_2O$ fertilization level was increased, but no significant difference was found between DM 10,617kg of $K_2O$180kgha and DM 10,845kg of $K_2O$ 240kg/ha. Crude protein and energy productivity of forages increased as $K_2O$ fertilization level was increased, and the increasing degree was highest between $K_2O$ 120kg and 180kg/ha fertilization. With increasing $K_2O$ fertilization, mineral contents tended to increase in K and WCa+Mg equivalent ratios, to decrease in Mg, but not to be regular in the other elements. Efficiency of $K_2O$ was relatively high at $K_2O$ 180kgha fertilization, which produced DM 12.2kg, net energy lactation 67.3M.J, starch equivalent 6.2kStE and total digestible nutrients 7.6kg per Ikg $K_2O$. Therefore, potassium($K_2O$) fertilization levels must be applied with I8Okg/ha for a reasonable management in mixed sward of alfalfa-grasses.