The temperature dependences of the linear permittivity ε' and the harmonic coefficient γ of composite materials obtained by embedding the ferroelectric NH4IO3 into the porous alumina (Al2O3)  matrix with a pore diameter of 60 nm were studied. It has been found  out that the phase transition is diffused and the Curie temperature shifts to a low-temperature region T ~ 25 K. The results obtained are interpreted within the framework of the phenomenological Landau theory and the Ising model. On the basis of these theoretical descriptions of the demonstration that the phase transition temperature in a nanocomposite is a consequence of dimensional effects. At the same time, the electrical interaction between particles in adjacent pores does not play an important role, due to the low spontaneous polarization of ammonium iodate and significant distances between neighboring pores.