Radicalism often triggers terror movements in Indonesia. The spread of radical ideology in Indonesia may support terrorism in the world. The clear picture of terrorism in Indonesia is crucial as the foundation for the government to take action against the radicalism. This study proposes the modeling of radicalism in Indonesia and the prevention of the dangers of radicalism by giving treatment. The model with the SERT compartment is used to describe the spread of radicalism in Indonesia. The SERT compartments are S = Susceptible, E = Extremist, R = Recruiters, and T = Treatment. This study uses the fifth Runge–Kutta as a numerical solution of the SERT model. The fifth Runge–Kutta has a higher level of accuracy compared to both the third and fourth Runge–Kutta. The results of modeling and simulations showed that the treatment for radicalism has a fairly high success. This success rate is indicated by the reproduction number R0< 1. The results of the modeling are that R0 value is 0.454. A low reproduction number can be interpreted that one radical person can spread to less than 1 person with a susceptible category. The low value of R0 (the decreasing of radical cases) is influenced by the number of arrested Extremist or Recruiters category and they got treatments. From the results, k = the success in overcoming radicalism cases is influenced by the length of the treatment process for people exposed to radicalism.