The influence of the ultrasonic welding process parameters, namely the force and the vibration amplitude, on the crystallinity degree at the welding interface of CF/PPS (carbon fibre reinforced poly(phenylene) sulphide) joints is investigated. Two different sets of parameters, one representing high force and high vibration amplitude (1000 N, 86.2 μm) and one representing low force and low vibration amplitude (300 N, 51.8 μm), were used in this study. The temperature at the centre of the overlap was measured using K-type thermocouples in order to obtain the cooling rate for each set of parameters. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) measurements were performed in order to determine the crystallinity degree of PPS at the welding interface. It was found that a force of 300 N and a vibration amplitude of 51.8 μm could obtain a PPS of a moderate crystallinity degree (14.6%) and an average PPS crystallite size of 41.3 Å, showing that it is possible to obtain a semi-crystalline welding interface by appropriately modifying the process parameters.