The effect of dietary L-carnitine and lipid on growth performance, feed utilization and body composition of Nile tilapia, Oreochromis niloticus were evaluated in two separate 2×5 factorial experiments as well as the effect of dietary L-carnitine on cold tolerance of Nile tilapia was evaluated in another experiment of the present study. The effect of L-carnitine was evaluated in five levels, 0, 300, 600, 900 and 1200 mg/kg diet at each of two lipid levels (5 and 10%), therefore ten isonitrogenous (30% CP) and isocaloric (3000 kcal ME/kg) diets were formulated and tested in three replicates with Nile tilapia (First experiment). Another experiment was applied by using the same previous diets (30% CP, 5% fat, 3000 kcal ME/kg diet) which formulated to contain increasing levels of L-carnitine, 0, 300, 600, 900 and 1200 mg/kg diet to study the effect of L-carnitine on cold tolerance of Nile tilapia (second experiment). In the present study, dietary L-carnitine at any level significantly increased all growth parameters (body weight, body length, weight gain, specific growth rate) and improved feed conversion and protein efficiency ratio of Nile tilapia. Compared to control group dietary L-carnitine supplementation significantly (P<0.05) increased the percentage of dress-out and flesh and decreased the percentage of by-products of Nile tilapia Also, increased dietary L-carnitine level decreased (P<0.05) fat content and increased (P<0.05) protein content of Nile tilapia. Results of the second experiment of the present study clearly showed that, the addition of L-carnitine to the diet of tilapia fish can substantially reduce their mortality during winter season. Fish fed the diet supplemented with 900 or 1200 mg/kg exhibited the best tolerance to cold water. In conclusion, dietary L-carnitine supplementation at all studied levels (300 to 1200 mg/kg diet) improved growth and feed utilization, decrease tissue fat and increased tissue protein and also increased survival rate of tilapia fry reared during cold season.