Parametric studies are made for the flexural behaviour of steel-concrete composite slim floor beams. After a short review on the slim floor construction, a numerical programme for the parametric analyses is briefly introduced. Eight parameters assumed to have influence on the composite action of the slim floor beams are chosen for the parameter variations. Totally 49 composite slim floor beams have been analysed from the commencement of loading to the failure of the beams. After discussions about the analysed results, conclusions are made for the strength and stiffness and for the influences of the parameters on the composite action of composite slim floor beams.