Two methods were compared with each other in relation to the deterioration of frying oil. In Method A a certain amount of oil was used for deep frying repeatedly, once a week for eleven weeks, without adding any fresh oil. On the other hand, in Method B, at each occasion of frying in eleven successive weeks fresh oil was added by the same amount as the loss at the previous frying. The amount of this addition was about 10% of the used oil in weight. Changes in the acid value, iodine value, viscosity, specific gravity, foaming degrees and fatty acid composition were measured with the oil used in respective ways. Also the aroma was evaluated by organoleptic tests. Our results confirmed that Method B was better than Method A.