The optical properties of composites consisting of inclusion particles of ‘metal core–J-aggregate shell’ structure within the dielectric matrix are investigated. Calculations of the frequency dependences of the dielectric functions of layered nanoparticles, the effective dielectric function, and the absorption coefficient of composites are carried out by using the classi-cal and dimensionally adjusted Maxwell-Garnett models of effective medium. As shown, one exciton and two hybrid plasmon–exciton modes are ex-cited in the investigated composite nanoparticles. The nature of oscillations of the real and imaginary parts of the dielectric function of layered particles in the infrared frequency range is explained. The approximate analytical model is constructed to determine the frequencies of hybrid plasmon–exciton resonances. As found out, these frequencies are significantly dependent on the metal content in the nanoparticle. The condition is defined, under which the first maximum of the imaginary part of the dielectric function of a nanoparticle falls into the infrared range of the spectrum. As shown, the frequencies of the hybrid modes of the nanocomposite depend significantly on the dimensions of the core and shell. The third maximum of the imaginary part of the effective dielectric function is located in the ultraviolet frequency range. As proved, the dimensional correction of the Maxwell-Garnett model is used, when the frequency dependences of the absorption coefficient of nanocomposite are studied, and the difference in the results of the calculation of the effective dielectric function is insignificant. The changes of the amplitude, the positions and the numbers of maxima of the imaginary parts of the dielectric functions of particles and composite as well as the absorption coefficient are traced, when the sizes and materials of the core and shell of the inclusion particles are changed. It is represented that, if the metal content is decreased in the inclusion particle, the maxima of the absorption coefficient of the composite are converged, and if the metal content is increased, then the maxima are moved apart.