Ferroelectricity was studied for the nanocomposites consisted of KDP loaded MCM-41 molecular sieves and porous glass with pore sizes 3.8 and 7 nm, respectively. Studies were carried out by NMR and dielectric techniques. The ferroelectric phase transition in the MCM-41 nanocomposite was shifted by 0.4 K to high temperature, while the ferroelectric transition in the porous glass nanocomposite was not found. The different ferroelectric behavior of the MCM-41 and porous glass nanocomposites with KDP was ascribed to the impact of the interparticle coupling. The ³¹P MAS NMR measurements showed that the isotropic chemical shift at room temperature was the same for nanocomposites and bulk KDP. This result confirmed the similarity of the crystalline structure of confined particles and bulk KDP. The chemical shift tensor parameters below the ferroelectric phase transition were calculated for bulk KDP and KDP confined to MCM-41 molecular sieves.