The study examined the determinants of modified audit opinion of listed non-financial firms in Nigeria. The objective of the study was to investigate whether audit firm size has significant effect on modified auditor opinion in non-financial firms in Nigeria. The secondary source of data collection was adopted in the study where the purposive sampling technique was used to select a sample size of ten (10) non-financial firms in Nigeria for the study. Least Square regression analysis was used in this study and the findings revealed that audit firm size has significant effect on modified auditor opinions in non-financial firms in Nigeria. The study concluded that the size of the board will influence the likelihood of a firm receiving modified audit opinion and firms with large boards were less likely to receive modified opinion due monitoring effectiveness of large boards. Finally, it was recommended that the size of the audit board should be properly partitioned to ensure a balanced opinion and adequate representation of the financial status of the firm.