The length weight relationships and feeding habits of juvenile and adult Cyprinus carpio and Oreochromis niloticus were studied from Lake Ziway, Ethiopia from April to August 2017. The juveniles were obtained using Beach seine net from three studying sites, and the adult fishes were purchased from catches of the landing fishermen. A total of 315 common carp (240 adult and 75 juveniles), and 365 Nile tilapia (170 adult and 195 juveniles) fish sample were collected ranging from 5.2 to 55 cm and from 2.5 to 30 cm TL, respectively. The corresponding TW ranges from 4.6 to 1610.2 g and 0.5 to 459.7 g, respectively. The relation between TL and TW of common carp was curvilinear with a strong relationship (R2=0.910 juveniles; R2=0.962 adults). The â€œbâ€ value was 2.704 and 2.909 for juvenile and adults of common carp respectively which were closer to the isometric growth value (b=3). Similarly, the length-weight relationship of Nile tilapia was also curvilinear and showed strong relationship (R2 =0.948 juveniles; R2=0.979 adults). The adult Nile tilapia had an isometric growth (b=2.908) while the juvenile showed allometric growth (b=2.523). From the total number of 75 juvenile and 240 adult of common carp, 4 (5.3%) and 60 (25%) fish samples had empty guts and the remaining 71 (94.5%) and 180 (75%) guts were observed with food, respectively. Whereas from 195 and 170 juvenile and adult Nile tilapia, 30 (15%) and 55 (26.5%) fish samples had empty stomachs while 165 (85%) and 115 (73.5%) were with foods, respectively. Fish samples that contained food in their stomachs were analyzed using frequency of occurrence and volumetric analysis methods. The major food items found in the stomach of juvenile common carp was zooplankton (34.24%), insects (26.34%) and detritus (25.82%) volumetrically while adult feeds on detritus (31.38%), macrophytes (30%) and insects (20%). The diet of adult Nile tilapia mainly depends on macrophytes (36.2%), phytoplankton (34.36%) and subsequently detritus (18.41%) while juvenile fed mainly on zooplankton (33.79%), phytoplankton (25.44%), insect (18.69%) and detritus (14.02%) volumetrically. In the diet of adult common carp, the contribution of insect was high from April to May while, detritus and macrophytes were high from June to August showing temporal variation in diet. The contribution of phytoplankton in the diet of adult Nile tilapia was high from April to May and low from June to August while, detritus and macrophytes were low from April to May and high from June to August. The contribution of zooplankton was high from June to August and low from April to May in the diet of juvenile Nile tilapia. However, there is no temporal variation in the contribution of insects in the diet of Nile tilapia except being relatively higher in June. The diet composition of this study shows that the two species are omnivore type which feeds on plant and animal origin. Schoener's index revealed significant dietary overlap between the two species, Î±= 0.84 (between juveniles) and Î±=0.63 (between adults). However, there was no significant dietary overlap within species between juveniles and adults of common carp and Nile tilapia (Î±=0.52) which indicate the presence of ontogenetic dietary shifts in both species. The presence of significant dietary overlap between the two species may be one of the causes for reduction of Nile tilapia stock in Lake Ziway. Further investigations are recommended for verification of the present findings and searching for alternative and complementary causes.