Dual-channel thin-film transistors (TFTs) show excellent effective mobility (μ _eff) and reliability, including negative-bias-illumination stability. To enhance the performance of these devices, post-thermal annealing in air is performed for 2 h at the low temperature of 250°C. To investigate the effects of low-temperature thermal annealing, indium–zinc-oxide/gallium–indium–zinc-oxide (IZO/IGZO) dual-channel TFTs with various IZO thicknesses are fabricated and examined. The observed parameters are μ _eff, saturation mobility (μ _sat), subthreshold slope (SS), and threshold voltage (V _TH). The interface quality may be improved by low-temperature thermal annealing, which was confirmed by the improvement in μ _eff and SS. However, these performance enhancements are intensively observed only in the channels with an IZO thickness below 10 nm. When the IZO thickness becomes 10 nm or more, the improvement in μ _eff due to low-temperature thermal annealing hardly occurs. This is because μ _eff is greatly influenced by the interface quality due to Coulomb scattering when the IZO thickness is <10 nm. However, for an IZO thickness above 10 nm, as phonon scattering also has a significant effect on μ _eff, the improvement in interface quality by thermal annealing is small. This is also supported by low-frequency noise measurements, which are sensitive to interface quality.