The disclosure relates to a process for the preparation of crystalline Bortezomib (Ia) as its monohydrate which is designated as crystalline Form-SB (Ia) and characterized by having water content ranging between 3.5 - 6.0 % w/w X-ray powder diffraction pattern comprising characteristic 2&thetas° peaks selected from the XRPD peak set of 5.6, 7.5, 9.8, 10.2, 11.3, 15.1, 18.0, 20.5, 21.5and 23.6 ± 0.20 2&thetas°, wherein peaks at 9.8 and 11.39 ± 0.20 2&thetas° are un-split and 100 % intensity peak is present at 5.6± 0.20 2&thetas°, DSC isotherm comprising the endothermic peaks ranging between 45 to 60°C (Peak -1) and 175 to 185°C (Peak -2) and IR absorption characteristic peaks approximately at 3387 cm-1, 3304 cm-1, 2953 cm-1,2927 cm-1, 2868 cm-1, 1627 cm-1, 1455 cm-1, 1400 cm-1, 1201cm-1, 1150cm-1, 1020 cm-1, 747 cm-1and 702 cm-1and Raman absorption spectra having characteristic peaks approximately at 3066 cm-1, 1583 cm-1, 1528 cm-1,1281cm-1, 1213 cm-1, 1035 cm-1, 1022cm-1and 1004 cm-1, wherein the process comprises the steps of: (a) combining the Bortezomib with an aliphatic ester solvent or a mixture of aliphatic ester solvent and water (b) raise the temperature upto about 40- 70°C (c) stir the solution at same temperature up to a time between 15 to 60 minutes (d) combine with aliphatic C6 to C7 hydrocarbon solvent (e) optionally maintain the mixture for 10-60 minutes (f) cooling the mixture upto about 10-40°C and (g) isolating the crystalline material. Said crystalline Form-SB of Bortezomib is used as an active pharmaceutical ingredient in pharmaceutical compositions for the treatment of cancer.
