The structure and electronic properties are investigated for Pt nanoparticles coated with octadecanethiol self-assembled monolayer. The increase in the octadecanethiol/Pt ratio from 0 to full coverage reduces the average particle size from 2.2 to 0.9 nm. The temperature-independent spin susceptibility rises upon the increase in the particle size, quantum size effect being suggested to govern the magnetism. The low-temperature susceptibility shows a large Curie-type divergence, which cannot be explained simply by the even electron state of Pt. XPS spectra suggest an electron deficiency in the interior Pt nanoparticles, which is brought about by charge transfer from nanoparticle to coating thiol monolayer. The ESR line width and the g-value deviation increase as the octadecanethiol/Pt ratio is elevated, which are associated with the enhancement of spin-orbit interaction at the interface between the interior nanoparticle and coating thiol monolayer. This change at the interface works to make first spin-lattice relaxation centers in the carrier scattering process.