Constant production of unstable molecules in the body beyond a limit result in severe damage of cell proteins, DNA, lipids etc. These set of species are free radicals which try to acquire stable configuration by snatching away the electrons from the surrounding molecules thereby resulting in a chain reaction. This leads to oxidative stress which is a starting point for various life threatening diseases. Coloured Petri-Net (CPN) on the other hand is a modelling tool which is suited and used for synchronization, concurrency and message passing as it is a graphical modelling language. This paper presents a modelling set up of reactions caused by free radicals using CPN tool.