The airline's revenue from each seat it flies a mile, a key industry metric, fell 0.5 percent in the quarter from a year earlier.
Revenue in the three months ended Sept. 30 was $5.27 billion, slightly lower than analyst estimates of close to $5.31 billion.
Investors will be eager to hear from Southwest executives about pressure from other no-frills carriers, as well as how the airline is faring amid the roll-out of basic economy fares on large full-service airlines. Southwest is also likely to provide more detail about fuel costs and its plans to start selling ticketsto Hawaii as early as next year.
Before the call, CEO Gary Kelly told CNBC that demand for Las Vegas flights fell in the wake of the shooting massacre at an outdoor concert there earlier this month.
"It's a revenue softness," he said, adding that demand is "gradually" recovering.