Feed capture in fish is based on a suction mechanism to ingest prey and feed. Then, particle size determines if it can be ingested by the fish or not depending on mouth size. Therefore, pellet size is especially important during the nursery period, notably the first time when fish larvae switch to complete feed. To identify proper pellet sizes for tilapia during the nursery period, we conducted an experiment to measure the mouth gape of red tilapia fingerlings for 78 days at ADM Aqua R&D Center. Tilapia fries of 5-day-old were stocked into 6 nursery tanks (400 L of water each) at a density of 300 fish/tank. Six fish were sampled twice per week in each tank until the fish reached 35 g. For young fish that were under 0.5 g, the mouth gape was estimated based on the linear regression equation between mouth gape (G) and body weight (W) of O. niloticus: (r2 = 0.96) (Yonas Fessehaye et al., 2005). For fish weighing greater than 0.5 g, the mouth gape was measured with a technical ruler and we established the new linear correlation for red tilapia: (r2 = 0.97). From the experiment results, we found that there is a remarkable gap between actual mouth gape and pellet sizes in current practices. More precise feeding guidelines will be discussed and proposed in our presentation.