The microstructural evolution of HSLA ISO 3183 X80M (API 5L X80) friction stir welded joints was evaluated with the goal of optimizing the joints fracture toughness. 12 mm thick plates fabricated using TMCP without accelerated cooling were welded with two passes using two different welding parameters with differentiated heatinputs. Optical microscopy, and scanning and transmission electron microscopy where used for the characterization. Both used parameters produced consolidated welded joints. The observed microstructures where formed by ferritic bainite, martensite-austenite (M-A), and dispersed carbides for both welding conditions. The higher heat-input stir zone also presented ferritic bainite, M-A, and dispersed carbides, providing excellent fracture toughness (CTOD). On the other hand, the lower heat-input stir zone presented degenerated upper bainite, upper bainite, coalesced bainite, M-A, and dispersed carbides, with low CTOD values.