The effect of Ni and Mo addition on the microstructures of a high strength low alloy (HSLA) steel under the thermo-mechanical control processing (TMCP) process is currently unknown, which is herein explored by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). As the Ni increase from 0.6 to 0.9 wt%, the volume fraction of bainitic ferrite increase from 30% to 50%, the average size decrease from 3 to 2 μm, and the size of the martensite–austenite (M–A) constituent decrease from 1.2 to 0.7 μm. The volume fraction of bainitic ferrite increases from 30% to 85%, and grain size increases from 3 to 5 μm with increasing Mo from 0 to 0.2 wt%. The size of M–A constituent decreases from 1.2 to 0.3 μm. Impact absorbed energy at −100 °C increases from 50 to 70 J as the Ni increase from 0.6 to 0.9 wt%, which decreases from 50 to 10 J with increasing Mo from 0 to 0.2 wt%. The improved impact toughness mainly results from the decreasing size of the grain and the M–A constituent with the increase in Ni from 0.6 to 0.9 wt%.