Aiming at improving the joint efficiency with enhanced cooling rate, the dissimilar joints of AA 7003-T4 and 6060-T4 were produced by underwater friction stir weld (UFSW), and microstructure and mechanical properties of the dissimilar joints were investigated in this study. It is found that sound and defect-free joints can be obtained through UFSW, while tunnel defects are formed with a high welding speed of 240mm/min. Due to the low temperature during UFSW, microstructure of the nugget is much finer than that of the normal FSW joint. The evolution of the precipitants is sensitive to the welding parameters of UFSW. With the welding speed increasing, more and phase remain due to the lower heat input. Compared with normal FSW, the heat-affected zone of the UFSW joint becomes narrow, and the soft region of the joint is closer to weld center according to the hardness distribution profiles. The largest ultimate tensile strength of the dissimilar joint is 183.9 +/- 1.4MPa and joint efficiency is 90.4%, which is higher than that of the normal FSW joint. The strength improvement is mainly attributed to the microstructure modification caused by water cooling.