A modular apparatus of fuel cell system includes a start burner, a reformer, an after-burner, and a heat exchanger. The start burner, the reformer, the after-burner, and the heat exchanger are disposed in a chamber. The start burner is surrounded by the reformer, and the after-burner is disposed on the start burner and surrounds the reformer. The heat exchanger surrounds the after-burner and the reformer.