Thai Airways fell into the red last year despite increased passenger numbers, load factors and new aircraft.
The carrier suffered a net loss of 2.11 billion baht ($67.41 million) for the 2017 fiscal year, blaming aircraft maintenance, an impairment loss and higher fuel prices.
Thai, which reported a profit of 15.14 million baht in 2016, missed analysts estimates of 2.6 billion baht in profits for 2017.
The downturn came even though passenger carryings rose by more than 10% to 24.6 million as international tourist arrivals to Thailand increased by 9.9% to 35.2 million.
Load factor of 79.2% was the highest in a decade and up from 73.4% a year earlier.
However, the airline warned that competition from low-cost carriers and upward trend of fuel prices were risks for the year ahead.
Thai carriers have been struggling to make the most of a boom in tourism to Thailand, with a 6% rise in tourists to 37.55 million expected this year.
Thai Airways expected to receive five new Airbus A350-900s this year to fly long-haul and regional routes.