NOVELTY - Preparation of linezolid crystalline Form III involves: providing a solution of linezolid in water at 80-95 degrees C, maintaining the solution under stirring at 80-95 degrees C, cooling the solution obtained in second step slowly at a temperature of below 35 degrees C to cause crystallization and recovering the highly pure crystalline Form III of linezolid from the reaction mass obtained in third step.    USE - For preparing linezolid crystalline Form III (claimed) which is used as antimicrobial agents, effective against a number of human and veterinary pathogens, particularly gram-positive aerobic bacteria such as multiply-resistant Staphylococci, Streptococci and Enterococci as well as anaerobic organisms and acid-fast organisms.    ADVANTAGE - The method is improved, cost effective method, is commercially viable and industrially advantageous processes for the preparation of highly pure linezolid crystalline Form III; is environmentally friendly processes for preparing linezolid crystalline Form III with high yield and high chemical and enantiomeric purity.    DETAILED DESCRIPTION - Preparation of linezolid crystalline Form III involves:    (1) providing a solution of linezolid in water at 80-95 degrees C, maintaining the solution under stirring at 80-95 degrees C, cooling the solution obtained in second step slowly at a temperature of below 35 degrees C to cause crystallization and recovering the highly pure crystalline Form III of linezolid from the reaction mass obtained in third step;    (2) providing a first solution of (S)-N-((3-(3-fluoro-4-(4-morpholinyl)phenyl)-2-oxo-5-oxazolidinyl)methyl)amine in methylene chloride, optionally adding a base to the first solution to form a second solution, combining the first or second solution with acetic anhydride to produce a reaction mass containing linezolid, separating the organic layer from the reaction mass, optionally subjecting the organic layer to carbon treatment or silica gel treatment to obtain a filtrate, optionally adding water to the obtained organic layer or filtrate to produce a reaction mixture and removing the methylene chloride solvent from the obtained organic layer, filtrate or reaction mixture by distillation or evaporation to produce the highly pure crystalline Form III of linezolid, and    (3) providing a suspension of linezolid in water at 60-80 degrees C, maintaining the suspension under stirring at 60-80 degrees C, optionally, seeding the suspension obtained in second step with linezolid Form III, cooling the suspension obtained in second or third step gradually to a temperature of below 35 degrees C and recovering the highly pure crystalline Form III of linezolid from the resulting mass obtained in fourth step.