The Indochina peninsula has been hypothesized to be in the Sunda block, a rigid block in SE Asia. However, its northern boundary is not obvious. To elucidate the boundary and the deformation on the peninsula, six permanent GPS sites were established in Thailand, and observations have been conducted since March 1998. Estimated velocities suggest that most of the Indochina peninsula is rigid and can be considered in the Sunda block. However, close examination suggest small but significant strain in the order of 10⁻⁸ exists in Thailand. Statistical test suggests that there is no significant differential motion between the Sunda block and the South China block. On the other hand, acute clockwise rotation of east Himalayan syntax indicates left lateral shear in the north of Thailand, which may be partially accommodated by left lateral motion along Mae Chan fault.