Southwest, based in Dallas, carries more passengers than any other airline in the United States.
Besides its Atlanta base, AirTran has hubs in Milwaukee and Orlando.
A merger between Continental Airlines and United Airlines will be completed at the end of the week, making the combined company, the world's largest airline, moving ahead of Delta. Delta held the top spot of two years after buying Northwest.
Southwest's purchase of AirTran, which requires shareholder and regulatory approval, should close during the first half of next year. The airlines will meld their operations in 2011.
Based on current operations, the new airline will 43,000 employees, serve more than 100 million customers and have operations in more than 100 different airports in the U.S. and near-international destinations.