Shares in Banpu jumped 3.7 percent to 392 baht after Thailand's biggest coal miner said it would buy back shares from the market for up to 6.2 billion baht ($208 million).
The main SET index was down 0.5 percent.
Banpu shares have fallen 5.1 percent so far this year, underperforming a 3.7 percent gain in the energy subindex .
Before the market opened, it reported a 57 percent drop in October-December net profit at 1.51 billion baht because weak global demand cut prices. Nine analysts surveyed by Reuters had forecast on average a net profit of 1.6 billion baht.
The company also said it planned an interim dividend of 9 baht per share.
1002 (0302 GMT)
(Reporting by Viparat Jantraprap in Bangkok; Editing by Prateek Chatterjee)