The present invention describes a novel process for large-scale purification of therapeutic grade quality of recombinant human GCSF from microbial cells, wherein the protein is expressed as inclusion bodies. The Inclusion bodies are solubilized and refolded under redox condition. The Redox condition is provided by using ascorbic acid, dehydroascorbic acid and reduced gluthathione. The process involves the novel use of aqueous two phase extraction step to purify refolded GCSF after removal of denaturant. After this step GCSF is further purified using chromatography techniques for removal of related impurities. The GCSF obtained has good purity and yields which are essential for a production scale process. The host cell related contaminants like proteins, DNA and endotoxins are reduced using the purification processes of the invention.