The use of microwave band gap structures for the non-destructive evaluation of material property is being probed in this paper. As a first step, we studied numerically, the appearance of a point defect mode within the band gap of a microwave band gap structure for the use as a tool to evaluate the dielectric constant of the material at the defect site. The simulations were carried out using the FEMLAB software. In a pure 10 £ 10 square lattice constructed with a material of dielectric constant (") 5.5 (in the form of a right circular cylinder), a point defect is created by replacing one of the rods with a geometrically similar but with different dielectric constant material. The appearance of the defect mode is governed by the refractive index contrast (defined as the ratio between the refractive index of the material at the defect site (ε) and the material of the lattice (nl). In this case, the refractive index contrast (nd=nl) was found to be between 0.85 and 1.28. Simulations were also done with the lattice material of dielectric constant 10 and the defect mode appeared for 1:18 < n d=nl < 0:84. For the contrast less than 1, defect creates the fundamental mode similar to TE01 mode whereas for the contrast greater than 1, next higher mode similar to TE11 appears. Once a defect mode appears, it moves towards lower frequency as the dielectric constant of the defect site is increased. In this paper, we propose to evaluate the dielectric constant of a material using the above procedure. This paper also proposes the construction of a novel frequency selector switch that has two line defects in a 10×20 square lattice structure constructed with material of dielectric constant 10. Copyright © (2008) by the Electromagnetics Academy.