The optimum welding condition for a 5056 aluminum alloy was investigated using a responce surface method. The optimum welding condition to yield maximum tensile strength at the weld was attained through a steepest ascent path. A second-order equation predicting the weld strength accurately was established. The successful weld showed 89.2% joint efficiency in tensile strength. The optimum welding condition was obtained using a comparatively small up-set burn off. Therefore, the economical consideration such as a loss of material was not needed to be taken into account. The observation of the weld showed that the friction layer formed at the friction interface disappeared in the successful weld.