Abstract: Cracking is one of the problems encountered frequently in cast-in-situ RC (reinforced concrete) floor slabs of mixed structure buildings. Based on the investigation on several practical projects and model experiments, nonlinear finite element models of two structural types were created and analyzed. The analysis results included the distribution and development of concrete principal tensile stresses, strains, slab cracks and steel rebars’ stresses, agreeing satisfactorily with the experimental results. The research conclusions point out the main reasons and factors of slab cracking, the form and range of cracks, key measures for controlling slab cracking, and the proper distribution of steel rebars in slabs.