Oreochromis niloticus (Nile tilapia) is an important fish in the ecology of tropical waters as well as aquatic systems in other subtropical regions. It is the most commercially important and preferred fish species in Ethiopia. It is also a candidate fish for aquaculture. In recent years, the fish community structure and ecosystem dynamics of the Lake Ziway have changed, but no studies have been carried out to assess whether the fishes have altered their diets in the lake. Indeed the food of Nile tilapia with respect to size was conducted from April to August 2017 in Lake Ziway, Ethiopia.A total of 365 Nile tilapia specimens (170 adults and 195 juveniles) were collected ranging from 2.5 to 30 cm TL and 0.5 to 459.7 g TW. Guts of 165 (85%) juveniles and 115 (73.5%) adults’ contained food items. Specimens that contained food items in their stomachs were analyzed using frequency of occurrence, volumetric methods and Schoener’s index. Nile tilapia originally known to be phytoplanktivorous has shifted to omnivorous feeding mainly on macrophytes. Volumetrically, the major diets of juveniles were zooplankton (33.79%), phytoplankton (25.44%), insect (18.69%) and detritus (14.02%) while the diet of adults were mainly macrophytes (36.2%) followed by phytoplankton (34.36%) and detritus (18.41%). Adult Nile tilapia which was a phytoplanktivorous has now shifted to macrophytes even though phytoplankton is the second dominant food item while juvenile mainly depends on zooplankton and insect larvae. Ontogenetic diet shift was observed in the present study demonstrating juveniles mainly feeding on animal based food items whereas adults depend on plant origin.