This paper is concerned with aging degradation of cable insulation used in airplane and power plants. Material degradation is associated with dielectric property. To prove availability of evaluating dielectric properties, first, a relationship between Q-factor and dielectric permittivity is identified. Secondly, to develop a method of identifying dielectric permittivity, mathematical model of the nondestructive testing for system is simply described by Maxwell's equations in two spatial dimensions. Thirdly, numerical scheme is developed using the finite-difference time-domain method (FDTD). Finally, parameters of dielectric property are estimated using information through numerical experiments by computational algorithm based on Markov Chain Monte Carlo (MCMC) method.