A facile and environmentally benign synthetic route to decorate preformed amine functionalized silica spheres (SiO2) by in situ formation of gold nanoparticles (Au NPs) at three different concentrations (1, 2, and 3 mM) of Au precursor (HAuCl4) is reported. UV-Visible absorption spectra of SiO2@Au (1, 2, and 3 mM) NPs showed a characteristic surface plasmon resonance band due to the presence of Au NPs and transmission electron microscopic images confirmed that the Au NPs were accommodated on the surface of the amine functionalized SiO2 spheres without aggregation. Herein, N-[3-(trimethoxysilyl) propyl] diethylenetriamine acted as both reducing and stabilizing agent for the Au NPs and no other protecting agents were used. Cyclic voltammogram recorded for the SiO2@Au NPs modified glassy carbon (GC) electrode showed a characteristic electrochemical response due to the presence of Au NPs on the electrode. Electrocatalytic reduction of hydrogen peroxide (H2O2) was carried out at the SiO2@Au (1, 2, and 3 mM) NPs modified GC electrodes, among which the SiO2@Au (3 mM) NPs modified GC electrode produced the highest catalytic current. Electrochemical sensing of H2O2 was performed using linear sweep voltammetry at the SiO2@Au (3 mM) NPs modified GC electrode with an experimental detection limit of 5 mu M.