The dielectric properties of eight amino acids (some of which are in the form of fine powder, and some, in the form of coarse powder) are determined for the first time in the MM wave range (80-116 GHz). The imaginary part of the complex permittivity of these amino acids ranges from values less than 0.001 to 0.075. The reflection and transmission coefficients of the samples do not exhibit any maxima and minima in the frequency band of 80-116 GHz. It is found that the irregularities in the transmission and reflection characteristics are due to the inhomogeneity of the samples, which inevitably arises when a long waveguide section with small cross-section is filled with coarse powders of amino acids. This inhomogeneity is responsible for the excitation of higher order modes in the waveguide filled with powder. The interference between these modes results in the above-mentioned irregularities in the transmission and reflection characteristics. Several repeated procedures of filling the waveguide section and measuring its transmission/reflection characteristics are performed. The averaging of the results of these measurements makes it possible to eliminate the irregularities in the characteristics. The obtained results give a possibility of using, for example, for determination of amino acid content in solutions.