Ultrasonic welding is a versatile welding technique for the assembly of thermoplastic composite structures. The research in this paper aims at improving the manufacturing efficiency and the weld attributes, featured by welded area, weld strength and failure mechanisms, of ultrasonic spot welded joints by adopting proper sonotrodes combined with process control techniques. To achieve this goal, the research was carried out in two steps. In the first part, single-spot ultrasonic welding was performed by using sonotrodes with various diameters. The influence of the sonotrode sizes on the welding processes and weld attributes of the as-manufactured welded joints was investigated. Based on the information gathered from the first part, a novel simultaneous spot welding technique to produce two welded spots within a single step was subsequently proposed. The results in both parts indicated that the increase of the sonotrode size could significantly improve the welding efficiency. The weld attributes were also effectively enhanced by adopting proper welding force.@en