A study was conducted to analyse the length-weight relationship and condition factor of Nile tilapia (Oreochromis niloticus) in Koka Reservoir, Ethiopia. A total of 132 fish samples were collected, measured and data analysed. The length and weight relationships of this fish species was found to be highly correlated (r = 0.964) and significant at the 0.01 level. The parameters ‘a’ and ‘b’ of the length – weight relationship were estimated using the formula W=aL^b, while the condition factor was calculated from the equation K=W/L³x100. The ‘b’ values in the length-weight relationships were; males (3.210), females (2.868) and both sexes combined (3.170). Males and combined sexes exhibited positive allometric growth, while the females exhibited negative allometric growth pattern. The mean condition factor (K) of the specimen produced male, female and combined sex values of 1.64, 1.70, and 1.66, respectively. All the K values were found to be greater than 1, mathematically indicating a healthy status and general well-being of Oreochromis niloticus in Koka Reservoir. The length-weight relationship and condition factor study was found to be a useful method in assessing the well-being and growth performance of the fish species in the Reservoir.