Southwest estimated that passenger revenue per available seat mile increased in between 8 percent to 9 percent, compared with January 2010. An available seat mile is one seat flown one mile.
Paying passengers flew 6.23 billion miles last month, compared with 5.50 billion miles in January 2010.
The airline expanded passenger-carrying capacity by 7.5 percent, to 8.19 billion available seat miles. Airlines increase capacity by adding flights or using larger aircraft.
With traffic rising faster than Southwest could add new seats, flights were fuller. Occupancy rose to an average 76 percent in January, compared with 72.1 percent a year earlier.
The average trip was slightly longer than a year ago, up to 909 miles from 864 miles last year.