Customary usage of oil at homes and rancidity of edible soybean oil by cooking frequency at homes and mass meal services were investigated. 80% of house wives bought the cooking oil by 1.8ι unit container and 70% of them read either the manufactured date or explanatory note for use. 85% of house wives kept oil in the storage case under sink or in the pantry chest, and 80% of oils were used once or twice and 20% used three times for cooking. Acid value(AV), iodine value(IV), peroxide value(POV), carbonyl value(CoV) and thiobarbituric acid value(TBAV) of fresh soybean oil were lower than standard level. In the rancidity by cooking frequency, the acid value, peroxide value, carbonyl value and thiobarbituric acid value increased significantly when oil was used once and iodine value decreased significantly when used once and twice at both mass meal services and homes. The level of the acid value, iodine value, carbonyl value and thiobarbituric acid value of oil used at mass meal services did not show significant difference from those of oil used at homes. But, the peroxide value of oil used thrice at mass meal services was significantly higher than those of homes.