Friction stir welding (FSW) is a solid-state welding process that is ideally suitable for joining materials with low melting point, such as aluminium alloys. In this paper, development of a fuzzy logic model to monitor weld strength and nugget hardness of friction stir butt-welded AA6063-T6 aluminium alloy plates is presented. FSW experiments were carried out with different process parameters in vertical spindle milling machine. The generated acoustic emission (AE) and machine spindle motor current during the FSW process were acquired with the aid of data acquisition system. A fuzzy logic model based on AE signal and motor current was developed to predict the weld strength and nugget hardness. The fuzzy model accuracy was validated with experimental data. The proposed model could be used for online weld monitoring of FSW process within a range of process parameters. © 2014 The Authors.