Abstract: The 30 mm thick plates of a Zr-Ti killed microalloyed low alloy high strength steel was welded utilizing gas sheilded arc welding.The microstructure in the heat-affected zone was studied by means of optical microscopy and electron microscopy.Many micron and sub-micron composite Zr-Ti oxides were observed and evenly dispersed in the heat-affected zone whereas no cluster-like MnS was observed.Oxide particles were precipitated on grain boundaries and in grain interior.They pinned the grain boundaries and inhibited the growth of austentite grains.The austenite did not coarsen remarkably.The acicular ferrite grains formed in the coarse-grained region of heat-affected zone effectively partitioned austenite grains and thus refined microstructures.The mechanical test of coarse-grained heat-affected zone showed that the Zr-Ti killed microalloyed steel plates had good low temperature toughness.