A feeding trial was conducted to evaluate the effect of supplementing MAXIMINERAL(72)(MM), a cornmercial product of Ca, Mg-bentonite, to the broiler diet. One thousand hatched rnale broiler chickens(Cobb strain) were randomly assigned to 4 dietary treatments; MM 0% (control), MM 1%, MM 2%, and MM 2% + tricalciurnphosphate (CaP) 0.5%. Each treatment had 5 replicates of 50 birds each. Birds were grown on floor and fed ad libitum for 5 wk. Through the whole period, weight gain and feed intake were not significantly different among treatments. However, weight gain of MM 2% + CaP 0.5% group was highest and orthogonal contrast showed that the feed intake during the period of 0~2 wk was significantly(p<0.01) lower in MM supplemented groups than the control. Feed efficiency (feed /gain) of MM supplemented groups were significantly better than the control. Feed efficiency of the control was significantly(p<0.05) higher than MM 1% and MM 2% + CaP 0.5% groups but not different from MM 2% group. Contents of ash, Ca and P of tibia were not significantly different among treatments but that of P tended to be low in MM 1% and MM 2% groups. There were significant (p<0.01) differences in plasma alkaline phosphatase (AP) activity among treatments. The highest AP activity was shown in MM 2% + CaP 0.5% group followed by the control, MM 2% and MM 1% group. It was concluded that supplernentation of MM at the level of 1% of broiler diet significantly improves feed efficiency and adjustment of P level rnay further improve the performance of the broiler chickens.