The crash happened in Ban Bueng District, Chonburi Province, about 114 kilometers (71 miles) southeast of Bangkok, when the van driver lost control and veered across the highway into oncoming traffic, said Lt. Col. Viroj Chiemcharas, deputy chief to the Ban Bueng Provincial Police Station.
In the van, 14 people were killed and one person survived, he said.
In the pickup truck, nine adults and two children were killed and one person survived.
The two survivors are being treated at local hospitals.
The cause of accident is still under investigation, police say.