This study examines the readability of audit reports to understand why investors do not utilize this information appropriately despite the increased amount of information following the 2018 audit report reform. Specifically, we investigate whether the key audit matters (KAM) readability of the new audit reports is differentiated for Big4 firms and whether the auditors’ perceived importance of quality control affects readability. Using a sample of public firms from 2020 to 2022, we find that Big4 audit report readability is lower than non-Big4 audit reports, and that the greater emphasis on quality control, as measured by the proportion of quality control labor costs, is associated with higher readability, and that the lower readability of Big4 audit reports is mitigated by the importance firms place on quality control. This suggests that investors have difficulty understanding new audit report information and that enhanced quality control can help improve readability. Therefore, to increase the value relevance of audit report information, accounting firms should consider KAM readability, and regulators should provide monitoring of their quality control from various aspects.