We investigated the effects of thermal annealing for high-density subgap states in amorphous In-Ga-Zn-O (a-IGZO) films by focusing on low-quality defective films deposited without O-2 supply (LQ films). It was found that most of the subgap states were thermally unstable and decreased dramatically by annealing at <= 400 degrees C in O-2. These defects (but with different shapes) were further reduced by 600 degrees C annealing, whose subgap states appeared similar to that of a-IGZO films deposited at an optimum condition (high quality, HQ films) and annealed at 300 degrees C. However, electron Hall mobilities and field-effect mobilities of their thin-film transistors (TFTs) were low for the LQ films/TFTs even annealed at 600 degrees C compared to those for the HQ films/TFTs. It implies that not only the subgap states but also heavier structural disorder deteriorated the electron transport in the LQ films. The present results also suggest that although a-IGZO deposition without O-2 supply is sometimes employed in particular for DC sputtering, supplying some O-2 gas would be better to produce good TFTs at lower temperatures. (C) 2016 Elsevier B.V. All rights reserved.