Shares in Thai Oil fell 10% in Friday's morning trading session at 05:00 GMT along a 1% decline in the benchmark index.
"In recapitalisation, there will be an equity offering of 10 billion baht ($304.04 million)," the company's Executive Vice President of Finance Wanida Boonpiraks told reporters on Friday.
Thai Oil would also sell stakes in electricity firm Global Power Synergy Pcl for 20 billion baht to parent firm, PTT Pcl, she said.
PTT already owns about a third of GPSC, and Thai Oil holds 20.78% of shares in GPSC.
The purchase for 15.38% of Chandra Asri would initially be financed by 18-month bridge loans from banks and PTT.
The announcement marks another in a series of major investments by units of Thailand's state-owned oil and gas giant, PTT Group.
Earlier this month, PTT Global Chemical Pcl agreed to buy German coating resins maker Allnex for 4 billion euros ($4.75 billion). GPSC has invested nearly $1 billion in renewable projects across India and Taiwan.
The investment in Chandra Asri would expand Thai Oil's position of a refiner to petrochemicals.
"It's a jump start," said company president Wirat Uanarumit, who added that it would have taken the company eight years to build its own olefin petrochemicals supply chain.
($1 = 32.8900 baht)
(Reporting by Chayut Setboonsarng; Editing by James Pearson)