Abstract: Unsteady solutions of the thermal flows through a curved rectangular duct of aspect ratio 2 is investigated numerically by using the spectral method over a wide range of the Dean number, 0≤Dn≤1000, with a temperature difference between the vertical sidewalls for the Grashof number, 1000≤Gr≤1500. The outer wall of the duct is heated while the inner wall is cooled. In the present study, unsteady solutions of the thermal flows for a single case of the Grashof number Gr = 1500 is investigated in detail and complete flow behavior, covering the wide range of Gr, is shown by a phase diagram. It is found that the steady flow turns into chaotic flow through periodic or multi-periodic flows for large or moderate Gr; for small Gr, however, the steady flow turns into chaotic state through various flow instabilities, if Dn is increased.