Caseinomacropeptide (CMP) from casein has been reported to have several physiological functions. The crude CMP produced from Na-caseinate using a commercial renneting enzyme was used in this study. The renneting enzyme hydrolyzed most of Na-caseinate to release crude CMP within 1 h. The crude CMP prepared from Na-caseinate by renneting enzyme and ultrafiltration was relatively pure according to SDS-gel electrophoresis and amino acid composition, and existed as a dimmer. The administration of crude CMP with a high fat diet in the high fat+CMP group reduced food efficiency ratio to the level of the control group. This result indicates that the crude CMP may be involved in inhibitory actions over digestive functions. Food intake and body weight gain in the high fat+CMP group were reduced compared to the high fat group by 10.9 and 21.5%, respectively, indicating that the crude CMP may lower an efficacy of digestive functions in gastrointestinal cells as well as suppress satiety by controlling a secretion of digestive hormones.