Terahertz (THz) radiation, as a low-risk and high-efficient radiation has attracted especial attention and development of compact and efficient THz sources for various applications is of considerable interest. Wavelength conversion in the standard optical fibers was first observed and investigated based on the nonlinear phenomenon known as the scalar modulation instability (SMI). Compared with standard silica-based optical fibers, silicon waveguides have advantages such as higher refractive index and lower absorption loss over the THz region. The SMI can be analyzed based on the FWM process and in order to satisfy the phase-matching condition, the dispersion characteristics of the silicon waveguide must be well engineered. To this end, the pump wavelength must be adjusted very close to the zerodispersion wavelength (ZDW) of the waveguide. There are many methods which can be used to engineer the dispersion curve; one of them is changing geometrical parameters of the waveguide. In this paper, a silicon waveguide based on the photonic crystal idea is designed for the first time, and using the SMI phenomenon, tunable wavelength conversion for generation of THz radiation is simulated. By changing the geometrical parameters such as the air hole diameter of the photonic crystal, dispersion and nonlinear characteristics of the waveguide are controlled and hence the generated THz radiation is tuned. The results show when the pump wavelength is set at λ<sub>p</sub>=5.60 μm in the normal dispersion regime and for the fixed lattice pitch of Λ= 4.80μm, as the air hole diameter is changing from d=4.60μm to d=0.86μm, the converted wavelength is tuned from λ=2.15μm to λ=326.17μm, respectively.