Police said the accident occurred on Ratchasima-Wang Nam Khiew Road in Pak Thongchai district at 12.30am.
Driver Pisamai Uttawichai, 38, and 17 passengers were hurt.
Two women were being treated at the district hospital for possible internal injuries. The rest suffered minor injuries and were soon discharged.
Pak Thongchai police chief Pol Col Kachen Setaputta said the driver dozed off and the van swerved and flipped over.