Sources of coherent synchrotron radiation (CSR) at THz wavelength region have been constructed at Electron Light Science Centre, Tohoku University, Sendai, Japan. Bunch train of extremely shorter electron pulse less than 100 fs will be provided by an injector linac employing thermionic rf gun. Bunch compression will be performed by means of velocity bunching in an accelerator structure. Radiation sources under development are a Halbach type planar undulator and an accumulator isochronous ring. The undulator employs large gap and long period length configuration to secure sufficient free space for propagation of radiation. A resonant frequency around 1 THz is achieved by employing a lower beam energy of ∼ 20 MeV. Since spectrum of CSR is depending on longitudinal bunch form factor, we have calculated CSR spectra for various conditions of the beam to evaluate the performance of the THz sources. Numerical simulation with multi-particle system has been carried out to understand the radiation power and angular distribution as well. The beam transport in the undulator is crucial for quality of the radiation because the beam energy is very much low relative to strong focusing power.