The aim of the present work was to prepare floating microspheres of acyclovir to prolong residence time in stomach and to sustain the release of acyclovir. Acyclovir loaded floating microspheres were prepared by double emulsion solvent evaporation method. The 32 full factorial design was applied to optimize the formulation. The resultant microspheres were evaluated for average particle size, percentage encapsulation efficiency, in vitro drug release and model fitting kinetics. Scanning electron microscopy, Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry were used to investigate the physical state of the drug in the microspheres. The particle size of microspheres was in the range of 275-340 µm. Percentage encapsulation efficiency was between 59%-77% w/w. Microspheres remained buoyant for more than about 12 h. The results of FT-IR spectroscopy and differential scanning calorimetry indicated the stable character of acyclovir in microspheres and also revealed absence of drugpolymer interaction. The in vitro drug release study showed that acyclovir release from the microspheres was slow and sustained for more than about 10 h. Drug release followed Korsemeyer-peppas model. The results of factorial batches revealed that the concentration of ethyl cellulose and stirring speed significantly affected drug encapsulation efficiency and particle size of the microspheres. Thus we can conclude that floating microspheres can successfully be developed to sustain the drug release.