Nonlinear normal modes of infinitely long functionally graded cylindrical thin shells with internal resonance in thermal environments were studied. The internal resonance condition was obtained. The nonlinear modulation equations with internal resonance were established using multiple scale method. The effects of gradient exponent, temperature and vibration energy on the frequency response of nonlinear normal modes were discussed. It is found that the nonlinear normal modes of system will undergo bifurcations with variation of detuning parameter, and the bifurcation value of detuning parameter depends on gradient exponent, temperature and vibration energy.