In order to research the mechanisms of effect of rare earth microalloying on the impact toughness of HSLA steel, oscillographic impact experiments have been tested by instrumented Charpy impact equipment. The microstructures including: tissues, characteristics of grain boundaries and volume fractions of the retained austenite have been analyzed by scanning electron microscopy (SEM), electron back scattering diffraction (EBSD) and X-ray diffraction (XRD). The solid solution quantity of lanthanum and precipitates of the niobium, titanium and vanadium were quantified by physics-chemical phase analysis. Sizes of the secondary phase precipitates were measured by small angle X–ray scattering (SAXS). The results indicated that, the toughness of the HSLA steel can be enhanced by addition of La into the steel, in particular to prevent cracks propagation. The mechanisms were as follow, the volume fraction of retained austenite, the quantity of high-angle and the low-Σ value coincidence site lattice (CSL) grain boundaries were increased. In addition, precipitates of niobium, titanium and vanadium were decreased and the sizes of the secondary phase precipitates were refined by addition of La into the steel.