The purpose of adding xylanase to wheat basal diet in this paper was to study the influences of xylanase on the quantity and composition of intestinal microflora, display the mechanism of xylanase promoting Oreochromis niloticus growth. Oreochromis niloticus were used as experiment objects in this study and their initial body weight was (100.39±17.83)g. Wheat basal diet was control. The tested diets were wheat basal diet added with different levels of xylanase (0.05%, 0.10%, and 0.15% respectively). Each treatment was devised with 5 repeats and each repeat had 40 male Oreochromis niloticus. The fish in floating cages were fed to satiation for 78 days. In this paper, we measured the amount of bacteria in intestinal of Oreochromis niloticus. Anaerobic bacteria were identified by BIOMERIEUX VITEKⅡ Automated Identification Systems and VITEKANA ID Kit. The results showed that the quantity and composition of intestinal microflora of Oreochromis niloticus were changed by dietary xylanase to wheat basal diets. The forgut quantity of aerobic bacteria of 0.10% and 0.15% xylanase groups were significantly lower than that of the control (P<0.05 respectively).The midgut quantity of aerobic bacteria of 0.05%, 0.10% and 0.15% xyanase groups were significantly lower than that of the control (P<0.05 respectively). The hindgut quantity of aerobic bacteria of the test groups were lower significantly than that of the control (P<0.05). The foregut quantity of anaerobic bacteria of 0.15% xylanase group was decreased by 76.85%, 75.62% and 75.83% compared with those of the control, 0.05% and 0.10% groups (P<0.05 respectively). The midgut quantity of anaerobic bacteria of 0.10% and 0.15% xylanase groups were lower significantly than those of the control and 0.05% xylanase group (P<0.05). The hindgut quantity of anaerobic bacteria of 0.05%, 0.10% and 0.15% xylanase groups were decreased by 13.13%, 48.30% and 60.62% compared with that of the control(P>0.05, P<0.05, P<0.05 respectively). The xylanase mainly influenced the proportion of Ent., Bac., Bact., Bif. and Lac.. The proportion of foregut Ent. of 0.05%, 0.10% and 0.15% xylanase groups were decreased by 13.16%, 29.61% and 49.34% compared with that of the control (P>0.05, P<0.05, P<0.05 respectively). The proportion of foregut Bac. of three test groups were significantly higher than that of the control (P<0.05). The proportion of Bac. in midgut and hindgut of 0.10% and 0.15% xylanase groups were higher than those of the controls (P<0.05). The proportion of foregut Lac. of 0.15% xylanase group was significantly higher than those of the control and 0.05% xylanase group (P<0.05 respectively). The proportion of foregut Bact. of the control was significantly higher than those of 0.10% and 0.15% groups(P<0.05 respectively). The proportion of midgut Bif. of test groups were increased significantly compared with that of the control(P<0.05). The proportion of midgut Lac. of 0.15% xylanase group was significantly higher than those of the control and 0.05% xylanase group(P<0.05). The proportion of midgut Bact. of 0.10% and 0.15% xylanase groups were decreased significantly compared with that of the control(P<0.05).