UPDATE 1-Southwest Air says traffic, unit revenue fell in Sept.

(Adds analyst note, shares)

Oct 5 (Reuters) - Southwest Airlines Co reported softer September traffic on Friday as fewer passengers boarded its planes, and its shares were off 1.5 percent as a widely watched revenue measure was below some analysts' expectations.

The Dallas-based discount carrier said traffic, measured by revenue passenger miles, fell 2.1 percent to 7.76 billion in the month from 7.93 billion a year earlier. Capacity, as measured by available seat miles, was down 1 percent.

Passenger revenue per available seat mile, an important measure called unit revenue, is estimated to have fallen 2 percent to 3 percent in September from the year before. In the third quarter, unit revenue likely increased about 1 percent, Southwest added.

Barclays analyst David Fintzen had forecast revenue growth of 2 percent to 3 percent for Southwest in September.

"Southwest was lower, although everyone else has been lower to some degree this month," he said.

Earlier this week, Delta Air Lines Inc said its unit revenue grew 0.5 percent in September, lower than it expected, partly because of lower walk-up traffic. Delta said bookings were solid and forecast a low-single-digit percentage improvement for October unit revenue.

Airlines typically see a seasonal drop in travel after the summer vacation season.

Southwest said its third quarter traffic was down 0.6 percent as capacity shrank 0.7 percent.

Load factor, or the percentage of plane seats filled, edged down to 77.0 percent in September from 77.8 percent a year earlier.

Southwest is looking to maintain its dwindling low-cost advantage against bigger rivals that have cut costs in bankruptcy.

Maxim Group aerospace analyst Ray Neidl said other challenges before Southwest included successfully integrating its 2011 acquisition of AirTran Holdings and reworking its business model in a changed industry environment.

Neidl cut his third quarter profit estimate for Southwest to 13 cents a share from 21 cents, citing softer ticket pricing and the company's third quarter unit revenue growth forecast.

Shares of Southwest fell 1.5 percent to $8.96 in afternoon trading while most other airline shares moved up in concert with the broader market.

(Reporting by Karen Jacobs, additional reporting by Alwyn Scott; Editing by Gerald E. McCormick and Marguerita Choy)