This study was conducted to evaluate the effects of different levels from Nigella sativa oil on growth performance and feed utilization and comparison of different nonlinear functions against linear model to describe the growth curve in Nile tilapia in order to select the best function. Nigella sativa oil was added at 1 ml, 2ml and 3 ml /1010 g diet to Nile tilapia diets based control diet and each diet was fed in two replicates during the period from 21/10/2014 to 7/4/2015.Winter experiment started at 27/1/2015 as the amount of feed decreased to be 5 g /tank every 2 days to study the effect of Nigella sativa oil on fish resistance to the decrease in bodyweight during winter season. Nigella sativa oil in Tilapia rations had significant effect on the different body weights. Specific growth rate did not affect significantly by elevated Nigella sativa oil levels except specific growth rate between 8 and 10 weeks (p=0.050), also, condition factor did not affect significantly except initial condition factor and after 12, 14, 16 and 22 weeks. Increasing the level of Nigella sativa oil in fish diet improved body length and body depth significantly. Furthermore, Nigella sativa oil had significant effect on feed intake and feed conversion ratio. In connection to non linear models and growth curve description, Weibull and Morgan-Mercer-Flodin equations gave the highest accuracies followed by Von Bertalanffy and linear regression. The present results showed that inclusion of Nigella sativa oil in Nile tilapia (Oreochromis niloticus) diets improved growth performance and feed utilization relatively. Regarding growth curve, Weibull and Morgan-Mercer-Flodin models were the best equations to describe growth curve.