The fire resistance of reinforced concrete structures is one of the important issues considered in their design. One of the major problems in analyzing concrete structures using finite element method is the constitutive law used to model their stress-strain behavior. This book presented a numerical method developed to analyze the thermal and structural response of reinforced concrete slabs in elevated temperatures. A finite element approach was used for both thermal and structural analyses, where the effect of high temperature on the mechanical and thermal properties of concrete and steel are taken into account. A model for the stress-strain relationship of concrete at normal temperature was proposed, where it considered four stages of behavior: elastic, elastic-damage, elasto-plastic, and elastic-softening-damage. Based on the principles of continuum damage mechanics, a constitutive law was developed by adding a parameter resembling the area and number of cracks propagating during loading. This model could represent the softening behavior of concrete for different specimen sizes.