Considering limit water sources for aquaculture in Egypt, biofloc system is one of the answers. Biofloc system limits water exchange during the aquaculture process. Successful application in tilapia culture was our concern. Therefore, the experiment was established to detect the optimal fish density within biofloc system. Three stocking density (100, 150, 200 fish/m3) were examined for the optimal density within biofloc system. Fish with average weight of (6.08±0.22) were randomly distributed in to plastic tanks of 150L water capacity in triplicate each to examine the different density on tilapia growth performance, feed utilization, water quality parameters, and water count of zooplankton. The best growth performance and feed utilization were recorded for100 fish/m3 (D1). With the increase of stocking density, a depression in growth performance was recognized. All water quality parameters (total ammonia nitrogen, No2, pH and temperature) ranged around safe values except for NO3. The highest water total zooplankton counts were recorded for (D1). The present results indicate that the optimal stocking density for Nile tilapia within biofloc system was 100 fish/ m3.