decreasing trend with the use of peppermint in diet (P = 0.118). In comparison with control diets, total count of microbialpopulations was increased in small intestine of birds fed peppermint (P = 0.005) through reduction in coliform bacteria andincreasing in LAB populations (P < 0.001, Table 5). Experimental treatments significantly affected the length and morpholog-ical parameters in small intestine and the ileum, respectively (Table 6). The smallest length of small intestine was observedin the positive control (62.6 cm) followed by negative control (64.8 cm) whereas the birds in peppermint groups exhibitedthe largest length of small intestine. Moreover, villus height, crypt depth, villus area, and villus height to crypt depth ratioin positive control were lower than peppermint groups. Regression analysis showed that the maximum villus area and CFUof LAB may be obtained with 28.9 (R2= 0.82) and 28.9 g/kg (R2= 0.93) of dietary peppermint, respectively (Fig. 2). However,the minimum CFU of coliforms may be achieved with 22.0 g/kg of dietary peppermint (R2= 0.50).