Both mathematical modeling and experiments have been conducted on the formation of the crater formed in a GMAW of aluminum alloy 6005-T4. Transient weld pool shape and the distributions of temperature and velocity were calculated by a three-dimensional numerical model. The final weld bead shape and dimensions were obtained. Corresponding experiments were conducted and in good agreement with modeling predictions. Metallurgical characterizations were also performed on the experimental samples. It was found that due to the fast solidification of the weld pool after the termination of the welding arc, there is no time for the molten metal to flow back towards the weld pool center and close up the crater. Thus a crater was formed at the end of the weld bead. Solidification cracking was formed at the center of weld crater. A "back-up" technique was proposed to allow extra molten metal to flow back to the crater and fill it up. The crater was successfully filled and the crater cracking was eliminated.