Abstract. Laser transmission welding (LTW) is a suitable process for assembling thermoplastic materials. This joining process is use to assembly most thermoplastics, but high-performance thermoplastics, such as polyetherketoneketone (PEKK), have received less attention until now. The present work deals with joining PEKK parts by LTW in amorphous over semi-crystalline state configuration. The optical properties of amorphous and semi-crystalline states were measured. The effect of the upper part thickness (2 or 4 mm) and those of the laser power reaching the interface was assessed through the determination of the heat affected zone (HAZ) dimensions and the mechanical resistance of the bonds. Single lap shear (SLS) tests were conducted to investigate the influence of the process parameters on the interfacial strength and to validate the weld quality. The highest LSS is obtained around 60 MPa. When increasing the laser power, the dimensions of the HAZ increase, and the mechanical strength decreases.