Police said the suspect, identified as Surapong Sae-Song, 22, was apprehended with the amphetamine pills in 10 fertiliser bags and ice in a truck bound for Bangkok.
The suspect was driving along the outer Chiang Rai-Therng route, heading for Nan to avoid the authorities. He was escorted by two cars.
Mr Surapong said he was hired at Bt300,000 to deliver the drugs to an agent in the capital.
Police said the drugs, produced by a group under Lt Col Yi Seng, were brought into Thailand via Doi Wawee in Mae Sruay district of Chiang Rai.
The drugs could fetch up to Bt250 million if successfully distributed, police said.