The present paper reports on a study of the synthesis of nanocrystalline high speed tool steel M50 powders (4.5% Mo, 4.0% Cr, 1.0% V, 0.8% C, balance Fe) by cryogenic high energy ball milling (cryomilling). Pre-alloyed M50 steel is spray atomized, and subsequently cryomilled in liquid nitrogen for 25 hours. Elemental Al powder is added prior to cryomilling to promote the formation of nanoscale Al 2 O 3 and AlN dispersoids to improve the thermal stability of the nanocrystalline M50 steel. High resolution transmission electron microscopy (HRTEM) reveals the formation of various carbides (V 8 C 7 , Fe 3 C, and FeC), oxides (Al 2 O 3 , MoO 3 , and V 3 O 7 ), and a nitride phase (AlN) during cryomilling. Following one hour of heat treatment at 1373 K (0.77 T m ), an average grain size of 70 nm was retained for the M50 steel powders.