The HTMS Sukhothai sank after water flooded its power controls on Sunday night, reports BBC.
Seventy-five people have been rescued so far, reports said.
"It's been more than 12 hours, but we will keep looking," a navy spokesman told the BBC.
The operation to rescue missing people is still going on.
"This has almost never happened in our force's history, especially to a ship that is still in active use," spokesman Admiral Pogkrong Monthardpalin told the BBC.