A novel, compact RF linac structure, the plane wave transformer (PWT), is studied. The PWT provides high accelerating field and high efficiency, and can be used to accelerate high-brightness beams. PWT linac prototypes with eight cells at S-band have been developed at UCLA and successfully used to accelerate an electron beam by more than 10 MeV in 40 cm. In this paper, we describe the principal properties of this structure, the electric parameters obtained from numerical simulation, and measurement results from microwave cold tests. The mechanical design for prototype linacs is also reported.