Instability, laminar-turbulent transition, and laminarization phenomena for slightly heated air flow in a curved square duct are studied by temperature fluctuation measurements for Reynolds numbers in the range Re = 10(3)-10(4). The method is based on the analogy between fluctuating temperature field and the fluctuating velocity component in the streamwise direction for forced convection. The fast Fourier transform (FFT) and the power spectrum density reveal the frequency and amplitude characteristics of the flow transition. The possibility of studying turbulence phenomena using passive scalar transport in developing turbulent flow is demonstrated. The stabilizing effects of centrifugal forces exist near the inner wall and the destabilizing effects dominate in the region near the outer wall. For flow near the outer region, Be temperature turbulence intensity increases to a peak value and then decreases to a fully developed value for turbulence with the increase of Reynolds number At Re = 10(4) turbulent flow exists in the core and outer regions but unsteady laminar flow persists In the inner region. Complementary secondary flow photographs are also presented for Re = 400-4 x 10(3). This study explores the possibility of using temperature fluctuation measurement to deduce the transition phenomena in fluid mechanics.