The electrical properties of nanocomposite SiAl z O x N y (Si) films containing Si nanoclusters embedded into amorphous SiAl z O x N y matrix have been studied by measurements of DC current−voltage and AC capacitance−voltage characteristics. Analysis of the results allowed us to conclude the existence of a negative dielectric constant. The temperature dependence of the negative dielectric constant has been obtained and analyzed. The negative capacitance has been revealed during measurements of capacitance−voltage characteristics at testing signal frequency of 2 kHz. The negative capacitance also points out the appearance of a negative dielectric constant effect. The qualitative model for explanation of negative dielectric constant based on peculiarities of SiAl z O x N y (Si) films polarization due to electron capture at Si nanoparticles−amorphous SiAl z O x N y matrix interface traps near cathode region has been proposed. In the case of AC C− U measurements, a negative capacitance is observed if conductivity current through the nanocomposite film is relatively high.