The purpose of the study was to get the valuable data for developing the new natural fat soaps which have an excellent biodegradation performance. Thus, natural fat soaps mixed with the two types of detergents (AOS and LAS) on the various concentrations were made and the biodegradation of the samples were analysed by Dissolved Oxygen method using active sludge. Also, the results were compared with the commercial synthetic detergents and market soaps. The results from the study were the followings: 1. The plant fat soap and the wasted oil soap with the concentration of 5 mg/l and 15 mg/l had an excellent biodegradation rather than animal fat soap. 2. There was little difference among samples with the concentration of 5 mg/l, but there was much difference among them with the concentration of 15 mg/l. 3. The periods for consuming oxygen of wasted oil soap mixed AOS and LAS was the fastest.