Abstract: In composite slab-on-girder floor systems the deflections and internal forces of concrete slabs are greatly influenced by the deflections of adjacent girders.The constitutive equations are derived according to the thin plate theory and solved with trigonometric series for the slab deflection to satisfy boundary conditions.The simplified formulae for curvature and moment in slab are derived and validated by comparing with the finite element solutions.The results of parameter analysis show that transverse curvatures and moments in slabs decrease as the slab to girder rigidity ratio and loads on girders increase.Even in one-way slabs,two-way curvatures should be considered when calculating moments in slabs.