Silver nanoparticles synthesized by a reagentless method involving only thermal treatment have been used in colorimetric assay for the detection of hydrogen peroxide. The silver nanoparticles were directly synthesized in polymethacrylate matrix (PMM) by method based on the reduction of the inorganic precursor AgNO3, which was pre-adsorbed by PMM from solution, through thermal treatment. These silver nanoparticles were characterized using absorption spectroscopy. It was shown that the silver nanoparticles in PMM (Ag – PMM) had ability for reduction of hydrogen peroxide with silver oxidation. As a result, the yellow color of Ag – PMM samples was gradually changed to transparent color depending on the hydrogen peroxide concentrations. Hence, the silver nanoparticles in PMM could be used as colorimetric sensor for hydrogen peroxide in aqueous medium.