Abstract: 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 20° 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?°, wherein peaks at 9.8 and 11.39±0.20 2?° are un-split and 100% intensity peak is present at 5.6±0.20 2?°, 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, 1201 cm?1, 1150 cm?1, 1020 cm?1, 747 cm?1 and 702 cm?1 and Raman absorption spectra having characteristic peaks approximately at 3066 cm?1, 1583 cm?1, 1528 cm?1, 1281 cm?1, 1213 cm?1, 1035 cm?1, 1022 cm?1 and 1004 cm?1.