Fluxes in submerged arc welding of high-strength low-alloy steels are not readily available, flux compositions are not clear and compositions are patented. This study aims at the design, development and optimization of flux for submerged arc welding of high-strength low-alloy steel. Extreme vertices design suggested by McLean and Anderson is used to formulate twenty-one fluxes to study the effect of flux constituents on tensile strength, percentage elongation, impact strength, diffusible hydrogen content and microstructure of the weld metal. Mathematical models for ultimate tensile strength, percentage elongation, impact strength and diffusible hydrogen content for welded specimens versus flux constituents have been developed. From the experiments, it is found that tensile strength and elongation are affected the most with Al 2O3 content, whereas CaO and CaF2 contents have significant effect on impact strength. Synergic binary effect of CaO·CaF2, CaO·MgO and Al2O 3·CaF2 mixtures is more than other binary mixtures on the mechanical properties. Developed regression models have been checked for adequacy using t-test for regression coefficients and analysis of variance (F-test) for whole regression equation. Finally, optimum flux composition giving optimum mechanical and microstructural characteristics is suggested. © 2013 IMechE.