The floating drug delivery systems (FDDS) have been developed to obtain prolonged and uniform release of drug in the stomach for the development of once-daily formulation. The objective of the present study was to develop once-daily sustained release FDDS of aceclofenac. The study involves preparation and evaluation of floatingmicrosphereswithmodel drug aceclofenac and polymer ethyl cellulose for prolongation of gastric residence time by solvent evaporation method. The microspheres remained buoyant in acidic medium containing surfactant for 8 hours in vitro. It was found that at higher polymer concentration, the mean particle size increased and the drug release rate decreased. The shape and surface morphology of microspheres characterized by optical microscopy and scanning electron micrographs, indicated that the microspheres were perfect spheres with porous surface aiding/showing good floating characteristic. The in vitro drug release study carried out in pH1.2 (0.1N HCl) for 8 hours showed slow release in buoyant condition and in small intestinemedium- pH6.8 buffer in a controlled release manner upto 20 hours. In vitro drug release studies showed that the drug release was faster in intestinal pH than as compared to gastric pH and drug kinetic analysis suggests that the drug release is a diffusion controlled release from floating microspheres. The in vivo antiinflammatory activity of optimized aceclofenac floating dosage formshowed retarded release in comparison to marketed formulation.