The net loss in the January-March period was put at 2.6 billion baht, chairman ACM Prajin Juntong said after a board meeting on Thursday.
It was a reversal of fortune from the same period of last year when the airline made a 8.29 billion baht profit. But the troubled airline still suffered heavy losses of about 13 billion last year.
Rising fuel costs and a sharp drop off in passengers mainly pushed the carrier into the loss column, he added.
He conceded there would be possible losses in the second and third quarters due to the spillover of the political turmoil which was deterring air travellers from coming to Thailand.
His concern was reflected in the cabin factor, or passenger load factor, which is an indicator measuring the capacity utilisation of planes, in April.
The cabin factor last month was 71.7%, a drop from 75.8% from April last year.
Tourists are not confident about coming to Thailand as many countries were still issuing travel advisory warnings for their citizens as elections remain uncertain, he added.
The loss came as no surprise as THAI acting president Chokchai Panyayong indicated it last month, but refused to give actual figures.
In the first quarter of the year, the number of THAI's Chinese passengers dropped by 25.8%, Japanese by 8%, and South Korean by 17%. They are the main tourists from East Asia.