Gold nanoparticle (AuNP) is a well-known biocompatible structure with several biomedical applications for labeling, heating, and sensing, besides delivery of drugs to cells. Similarly, platinum nanoparticles (PtNPs) have important applications in medicine. Traditional applications of these nanoparticles in medicine/biomedicine depend on their physical-chemical properties. In this work, a preliminary study of the antioxidative properties of AuNP and PtNP was performed using electron spin resonance (ESR) spectroscopy. Antioxidant activity against DPPH (1,1-diphenyl-2-picrylhydrazyl) radical was found for both nanoparticles, but the PtNP was more reactive than the AuNP to reduce the DPPH ESR signal. The decay time for the signal intensity was T = 3.1 � 0.1 min�1 for AuNP and T = 1.80 � 0.07 min�1 for PtNP.