About 370,000 litres of diesel were found on four boats modified for smuggling by a Navy-Marine Police team on Saturday in Chanthaburi.
Pol General Somyos Phumphanmuang, deputy National Police chief, said yesterday that patrol boats launched the raid at 8pm on a pier following several months of intelligence gathering.
The boats and cargo worth over Bt70 million were seized and 11 men arrested.
The suspects were identified as Chaiyan Suksa-nguan, 45, of Laem Sing district and 10 Cambodian crew.
Chaiyan allegedly confessed to having brought foreign diesel to the wharf many times for sale to fishing boats trawling off the Chanthaburi and Trat coasts.
He was paid Bt30,000 for each delivery.
Somyos said the government had allowed diesel to be sold free of customs and excise taxes to fishermen operating 12 nautical miles from shore, but the sale of untaxed oil onshore was illegal.