In work the questions connected with diffraction of the microwave oven - radiation on periodic structures of waveguide type are considered. The approach for assessment of reflective characteristics of the absorbing material based on assumption that according to the law of energy conservation all power falling the microwave oven - radiations on the absorbing material is distributed on two components, one of which characterizes reflection of electromagnetic radiation from the surface of material is offered and is defined by wave resistance on limit of the section "environment-the absorbing material", and other part, submitting to the law of diffraction of electromagnetic waves on not ideally carrying out closed surfaces of waveguide type, takes place in a lattice, is partially absorbed in the thickness of material is reflected from an object (a metal substrate) and at the exit is summarized from the first component. Feature of the offered technique are the settlement ratios allowing to define rational values of dielectric permeability of material, conductivity of elements of a lattice and also the period and thickness of a lattice causing achievement of "a semi-infinite layer" at which the minimum value of coefficient of reflection of material is observed and it does not depend on existence or lack of an object (a metal substrate) any more.