Three-dimensional non-linear finite element analysis till failure is performed on one hundred and thirty eight simply supported composite beams, to investigate the effective width of composite slab acting with exterior and interior steel beams. Full interaction is assumed between the used steel hot-rolled I-section and the overlaying composite R.C. slab with corrugated steel sheet. The ribs of corrugation are oriented perpendicularly to the steel beam longitudinal axis. Many variables are studied such as slab rectangularity, boundary conditions, case and type of loading, concrete layer thickness, beam span-depth ratio. Any other structural feathers of composite beams are considered constants. Interesting results, conclusions and recommendations, which may be useful for designers, comparison with the famous codes of specifications are presented in details.