Mechanical properties of wood dowel welding were studied using untreated and copper chloride (CuCl2)-treated wood dowels. The effect of the welding time (3 s, 5 s, and 7 s) was also studied. The treated wood dowels with a welding time of 3 s had the best pullout resistance. Fibers covered with black molten material generated by the high friction temperature were found at the welding interfaces. For the untreated groups, the degree of crystallinity of the welding interfaces was higher than that of the wood dowel. For the treated groups, the degrees of crystallinity for the welding times of 5 s and 7 s were lower than that of the wood dowel. By extending the welding time, the degree of crystallinity decreased. A thermogravimetric (TG) analysis was used to detect changes in the wood components. The hydrolysis of cellulose and hemicellulose occurred during immersion. The analyses illustrated that pyrogenic decomposition of the wood components occurred during the wood dowel welding process. For the treated groups, the degree of pyrolysis was higher than that of the untreated groups for the same welding time. An increased welding time also promoted pyrolysis during the welding process.