Secondary hardness due to carbide particles precipitation is a characteristic feature of highspeed steels (HSSs). The most dominant types of carbides responsible for this strengthening effect are MC and M2C ones, other species (M 6 C, M 3 C, M 2 3C 6 ) also being reported. They are typically within 5–30 nm range of size, of elongated shape and reach 3 % volume fraction. In the investigated niobium-titanium HSSs it was found that relatively large M 23 C 6 type carbide particles provide a secondary hardness twice the value encountered in commercial grades produced conventionally or by powder metallurgy. The higher effectiveness of the M 23 C 6 carbides was attributed to their higher volume fraction (approx. 5 %) and favourable plate-like shape. The role of the carbide forming elements in the experimental steels' crystallization and tempering was discussed.