A preparation method for gadolinium compound (GdC) nanoparticles coated with silica ($GdC/SiO_2$) is proposed. GdC nanoparticles were prepared with a homogeneous precipitation method at $80^{circ}C$ using $1.0{ imes}10^{-3}$ M $Gd(NO_3)_3$, 0.5 M urea and $0-3.0{ imes}10^{-4}$ M ethylenediarinnetetraacetic acid disodium salt dihydrate (ETDA) in water. As a result of preparation at various EDTA concentrations, GdC nanoparticles with a size as small as $40.5{pm}6.2$ nm, which were colloidally stable, were prepared at an EDTA concentration of $2.0{ imes}10^{-4}$ M. Silica-coating of the GdC nanoparticles was performed by a St$ddot{o}$ber method at $35^{circ}C$ using $1.0-10.0{ imes}10^{-3}$ M tetraethylorthosilicate (TEOS), 11 M $H_2O$ and $1.5{ imes}10^{-3}$ M NaOH in ethanol in the presence of $1.0{ imes}10^{-3}$ M GdC nanoparticles. Performance of preparation at various TEOS concentrations resulted in production of $GdC/SiO_2$ particles with an average size of $106.1{pm}11.2$ nm at a TEOS concentration of $5.0{ imes}10^{-3}$ M. The gadolinium (Gd) concentration of $1.0{ imes}10^{-3}$ M in the as-prepared $GdC/SiO_2$ particle colloid solution was increased up to a Gd concentration of 0.2 M by concentrating with centrifugation. The core-shell structure of $GdC/SiO_2$ particles was undamaged, and the colloid solution was still colloidally stable, even after the concentrating process. The concentrated $GdC/SiO_2$ colloid solution showed images of X-ray and magnetic resonance with contrast as high as commercial Gd complex contrast agents.