Phrae police said the incident happened at 03.00 am today near Kilometre marker 262 on the highway from Phrae to Nan.
The bus with 23 passengers on board and destination in Mae Sai district of Chiang Rai skidded off the highway when it reached a curve at the time when there was rain.
The bus lost control, overturned and fell into the side the highway.
All the passengers were injured and were later admitted to Phrae hospital.
But most suffered slight injuries and were all discharged after primary treatment.
The driver was booked for questioning.