In order to avoid the instability of arch in thermal environment, the nonlinear in-plane instability of a fixed circular arch under a central radial concentrated load in thermal environment is studied in this paper. The nonlinear equilibrium equations and buckling equilibrium equations are derived by the minimum potential principle. The theoretical solutions of the limit buckling and bifurcation buckling loads were then obtained and verified by the ANSYS finite element results. The nonlinear limit point instability and the bifurcation instability behavior of the arch was investigated. The results show that the temperatures have a significant effect on the nonlinear instability behavior of an arch. The limit instability loads and bifurcation instability loads increase with the rise of the temperature. The upper and lower limit point instability loads decrease with a decrease of the modified slenderness. It is also found that the maximum temperature difference for bifurcation bucking decreases rapidly with the increase of the slenderness.