The purpose of the present study was to systematically investigate some of the important physicochemical properties of Aceclofenac loaded micro particles.Aceclofenac is a potent analgesic, anti-pyretic and anti-inflammatory agent used in the management of moderate-to-severe pain and in rheumatoid disorder, rheumatoid arthritis and ankylosing spondylities. Almost all drugs are marketed as tablets, capsules or both. Prior to the development of these major dosage forms, it is essential that pertain fundamental physical and chemical properties of the drug molecule and other divided properties of the drug powder are determined. This information decides many of the subsequent events and approaches in formation development.A per oral extended release micro particles of aceclofenac was prepared by emulsion solvent evaporation method using polymer ethyl cellulose as well as eudragit RSPO and dispersion carrier as Aerosil. So before selection of excipients, the Preformulation study of drug aceclofenac is completed for successful formulation of per oral extended release micro particles. Preformulation studies included solubility, pKa, dissolution, melting point, assay development,, stability in solid state; bulk density, flow properties, excipient compatibility, entrapment efficiency, release profile of micro particles and Marketed brands of modified release product were investigated.