This invention relates to a micronizing device for fluid jet mills which presents a containing body (2) internally delimiting a substantially cylindrical grinding chamber (3), a plurality of nozzles (18), each presenting a mouth (22) opening onto a radially internal side wall (8) of the grinding chamber (3), a supply duct (9) for material to be micronized, opening into the grinding chamber (3) and at least one injection duct (12) for pressurized fluid, in fluid communication with said nozzles (18). The nozzles (18) present a pressurized fluid injection direction (X-X) which is tangent to an imaginary circle included inside the grinding chamber (3). The radially internal side wall (8) presents, at each nozzle (18), a first portion (23) which is substantially perpendicular to the injection direction (X-X) of the respective nozzle (18).