The present invention provides process for preparation of crystalline Bortezomib (Ia) as its monohydrate which is designated as crystalline Form-SB 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, 21.5 and 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&minus1, 3304 cm&minus1, 2953 cm&minus1, 2927 cm&minus1, 2868 cm&minus1, 1627 cm&minus1, 1455 cm&minus1, 1400 cm&minus1, 1201 cm&minus1, 1150 cm&minus1, 1020 cm&minus1, 747 cm&minus1 and 702 cm&minus1 and Raman absorption spectra having characteristic peaks approximately at 3066 cm&minus1, 1583 cm&minus1, 1528 cm&minus1, 1281 cm&minus1, 1213 cm&minus1, 1035 cm&minus1, 1022 cm&minus1 and 1004 cm&minus1. The invention also provides the use of said crystalline Form-SB as active pharmaceutical ingredient in pharmaceutical compositions for the treatment of cancer.
