Fires have a highly destructive effect on steel structures. Steel-concrete composite elements have better fire resistance than bare steel, but the composite behavior differs a lot in fire than at normal conditions. This paper studies the flexural behavior of steel-concrete composite simply supported girders in fire condition. A three dimensional nonlinear thermalstructural finite element model has been established using COSMOSM software package to evaluate both girder structural and thermal responses in fire condition. Temperature distribution, mid-span deflection, and stresses in both steel beam and concrete slab have been introduced as functions of time. Results of the finite element model have been verified against experimental work and analytical models found in the available literature. The proposed model proved an accepted correlation with the available experimental work. A parametric study based on the outputs of this model has been performed to study effects of variations in various structural parameters on the behavior of composite girders. Practical conclusions about fire endurance, recommended load ratios, and best composite configurations have been extracted.