Used / secondhand vehicles need to obtain an import permit from the Foreign Trade Department of the Ministry of Commerce. For the vehicles with the weight less than 3500 Kgs., the importer needs to obtain an import permit from the Industrial Standard Institute as well.
New vehicles of all types not yet registered abroad are allowed to be imported without applying for an import permit from the Ministry of Commerce. However, for the vehicles with the weight less than 3500 Kgs., an importer needs to obtain an import permit from the Industrial Standard Institute.
So for permanent import of used bikes two import permits, for new ones just one! The reason for many bikes being held at customs is the fact that these bikes arrive without the required import permits, so I am told. And I was also told that the permit from the Ministry of Commerce is actually the more difficult one to obtain.
The Thai Industrial Standard Institute is under the Ministry of Industry.