This study aims to investigate the relationship between internal control weakness and earnings management and tax avoidance related book-tax differences. The sample of this study consists of the firms listed on Taiwan Stock Exchange and Taipei Exchange (GreTai Securities Market) during 2006 and 2015. The empirical results present that internal control weakness is positively related to earnings management- and tax avoidance-related book-tax differences, suggesting that the firms with internal control weaknesses potentially conduct more earnings managing and tax avoiding arrangements than the firms without internal control weaknesses do. The findings are consistent using different ways to measure internal control weaknesses and book-to-tax differences.