Traffic fell to 9.23 billion revenue passenger miles from 9.38 billion in the year-ago period, the company said. A revenue passenger mile is an industry unit measuring one paying passenger flown one mile.
United blamed 19 severe weather days in the month for its performance. The company defines a severe weather day as one when more than 10 percent of its flights are affected by weather, air traffic control delays or cancellations.
UAL said United's two largest hubs, O'Hare International Airport and Denver International Airport, were among the three worst-performing airports for the month as unfavorable weather caused ground-delay issues. The problems at the two airports led to about 1,000 U.S. flight cancellations.
Capacity dipped 0.1 percent to 11.76 billion available seat miles from 11.77 billion in the previous year. United lowered its mainline capacity forecast due to its December weather problems. The company said it now expects mainline capacity will be off 1 percent year-over-year compared with a previous forecast for a 0.8 percent decline.
December occupancy dipped to 78.8 percent from 79.6 percent.
United operates more than 3,300 domestic and international flights a day.