Ultrashort, e.g., picosecond or femtosecond, electron beams are useful for electro-magnetic radiation production in terahertz (THz) range due to the inverse of 1 ps bunch length corresponding to the frequency of 1 THz. The electron beams were generated by a photocathode radio-frequency (RF) gun linac. Electron beams with energy of ̃4 MeV at the RF gun exit were accelerated up to ̃30 MeV in a linac. The accelerated electron beams were compressed by a magnetic bunch compressor, resulting in femtosecond pulse width. THz radiation of the order on 0.1 THz based on coherent Cherenkov radiation (CCR) using hollow dielectric tubes and Smith-Purcell radiation (SPR) using a metallic grating or metasurface were reported.