Ultrasonic Welding is a fusion bonding method that can create high-strength thermoplastic composite joints at very fast speeds. This makes it a very promising method for use in the aerospace industry, though further research is required to mature it. One topic of interest is how the crystallinity of the polymer matrix in the joint is affected by the ultrasonic welding process. This thesis specifically investigated the degree of crystallinity of carbon fibre reinforced lower melting polyaryl ether ketone (LM PAEK) composite. An experimental methodology was developed to measure the temperature at different locations through the thickness of the joint, and extract samples for Differential Scanning Calorimetry analysis. The calculated degree of crystallinity was related to the temperature evolution for different locations in the joint and variation in the welding force and amplitude.