In this paper, results of vibration welding of glass fibre-reinforced nylon 66 are presented. The hot air blower was used for preheating of the welded surfaces before the welding process. The aim of the research was determination of the influence of the thermal conditions of vibration welding process on the quality of joints, including weld microstructure, the content of the crystal phase, orientation of the glass fibres in the weld and the tensile strength. Influence of the welding conditions on the weld microstructure and the orientation of the glass fibres was determined during the light microscopic examinations as well as in the scanning electron microscopic examinations. A melting temperature and the content of the crystal phase of welds were tested using differential scanning calorimetry (DSC). Fitting of the mathematical function describing penetration curve was performed, and calculations of the heat power were made. Comparison of results of vibration welding of nylon composite conducted with and without preheating was carried out. Results indicate that the use of preheating during vibration welding process eliminates the solid-state friction in the welding process and leads to the significant improvement of the welds quality at the shorter weld time.