AA 6061-T6 aluminium alloy (Al-Mg-Si alloy) has gathered wide acceptance in the fabrication of light weight structures requiring a high specific strength and good corrosion resistance. Compared with the fusion welding processes that are routinely used for joining structural aluminium alloys, friction stir welding (FSW) process is an emerging solid state joining process in which the material welded does not melt and recast. Joint strength is influenced by the grain size and tensile strength of the weld nugget region. Hence, an attempt was made to develop empirical relationships to predict grain size and tensile strength of friction stir welded AA 6061-T6 aluminium alloy joints. The empirical relationships are developed by response surface methodology (RSM) incorporating FSW tool and process parameters. A linear regression relationship was also established between grain size and tensile strength of the weld nugget of FSW joints.