The company's net income rose to $823m (Â£580m) in the fourth quarter of 2015, up from $28m during the same period a year earlier.
The airline's fourth-quarter fuel bill fell 36% compared with a year earlier, saving the airline $912m.
United did suffer a 3% decline in total revenue from $9.3bn to $9bn, caused by slightly lower fares.
"We improved our operational performance, continued to invest in our products and services and achieved record financial performance," said Brett Hart, United's acting chief executive officer.
In October, United Airline's chief executive Oscar Munoz suffered a heart attack. He received a heart transplant in January. Investors were told Mr Munoz was "feeling great" and would be back by the end of the first quarter.
On Thursday, United also said it would expand its fleet by the middle of 2017, adding 40 new Boeing aircraft. It plans to introduce cheaper, but more restrictive ticket options by the middle of this year.
The company has struggled to cope with a complicated 2010 merger with Continental Airlines. It has faced technology woes, declining passenger numbers and a scandal concerning the New York and New Jersey Port Authority.
United Airlines saw its fourth-quarter income boosted by low oil prices, the company has reported.