We evaluated in the present study the effect of deltamethrin resistance on the fitness cost of the filed populations of Anopheles labranchiae. A susceptible population was used as reference to do different comparisons. We selected the most resistant larvae population collected from northern Tunisia. Eggs were used for study of life history traits including developmental time, larvae mortality, fertility, hatchability and adult sex-ratio. Our results showed that deltamethrin resistance affected negatively (p < 0.05) the developmental time with the median range of 70 h, mortality with the rate of 7 folds in resistant population and hatchability which are lower than in susceptible population. Whereas, no significant differences were detected in adult sex-ratio and fertility of the two studied populations. Our results could help to determine the evolution of population dynamics of the resistant studied population in the areas where insecticide resistance is reported and resistance management is needed. Keywords: deltamethrin, Insecticide resistance, Fitness cost, Anopheles labranchiae, Malaria vector, Tunisia