The contribution of nano-scale particles observed using Atom Probe Tomography in an increase of yield strength of conventional and advanced HSLA steels was studied. The advanced HSLA steel showed higher yield strength than conventional HSLA steel. There were two types of carbides, which primarily contribute to an increse in yield strength of conventional HSLA steel: (i) coarse TiC with average size of 25+-5nm and (ii) fine TiC with average radius of 3+-1.2nm. The presence of two types of carbides was found in the microstructre of advanced HSLA steel: (i) nano-scale Ti0.98Mo0.02C0.6 carbides with average radius of 2.2+-0.5nm, and (ii) C19Cr7Mo24 particles with an average radius of 1.5+-0.3nm. The contribution of precipitation hardening in the yield strength of advanced HSLA steel due to the nano-scale particles was 174MPa, while this value in the conventional HSLA steel was 128MPa.