The application of high strength low alloy (HSLA) steels has been limited by unavailability of suitable joining and filler metals in submerged arc welding (SAW) processes. The present work aims at the design and development of flux for Submerged Arc Welding of HSLA steel. In the work L8 array of Taguchi Design is used to formulate eight types of fluxes to vary basicity index (BI) from 1.26 to 2.81 and to study the effect of flux constituents and basicity index on tensile strength, microhardness and microstructure of the weld metal. Empirical models for ultimate tensile strength and microhardness at the centre of weld versus flux constituents and basicity index have been developed. From the experiments it is found that ultimate tensile strength increase with increase of basicity index with minimum at 1.26 increases upto 2.33 and then further decreases whereas opposite in case of microhardness which is highest at 1.26 and minimum at 1.9. Increase of CaO in the flux increases ultimate tensile strength but microhardness remains unaffected whereas increase of SiO2 decreases ultimate tensile strength but microhardness remains constant. Microhardness decreases critically with increase of CaF2. © (2013) Trans Tech Publications, Switzerland.