Disclosed herein is a process for preparing crystalline Pemetrexed dipotassium (I) Form-SP9 characterized by X-ray powder diffraction pattern comprising at least 5 characteristic 2&thetas° peaks selected from the XRPD peak set of 5.0, 12.5, 17.2, 20.8, 22.4, 25.7, 26.9 and 27.9 ± 0.2 2&thetas°, comprising the steps of: (a) providing a solution of Pemetrexed dipotassium in water (b) cooling the reaction mixture to temperature below 10 °C (c) addition of a polar organic solvent to the reaction to obtain a reaction mass, wherein polar organic solvent is selected from methanol, ethanol, n-propanol, isopropanol, n-butanol, amyl alcohol or isoamyl alcohol (d) treating the reaction mass with a secondary solvent, wherein secondary solvent is a (C3-C6) ketone solvent, selected from acetone, butanone or pentanone and (e) isolating the crystalline material Pemetrexed dipotassium (I) Form-SP9.
