The aim of this study was to develop an evaluation scale for use as a supplementary tool for the diagnosis of oppositional defiant disorder (ODD). The subjects were 98 Japanese children (91 males and 7 females), aged 6-15 years, diagnosed with attention deficit/hyperactivity disorder (ADHD) or ODD. Internal consistency, test-retest reliability, concurrent validity and divergent validity of the oppositional defiant behavior inventory (ODBI), an evaluation scale of oppositional defiant tendency, were examined. Cronbach's alpha coefficient of the ODBI was 0.925. The correlation coefficient between the test and the retest was 0.820 (p < 0.0001). Both the ODBI scores (test and retest) were correlated with the number of items that matched the ODD diagnostic criteria of DSM-IV (r = 0.660, 0.659, p < 0.001), and with the ODD-scale of Disruptive Behavior Disorders Rating Scale (r = 0.725, 0.654, p < 0.001). Compared with the ADHD group or controls, the ADHD and ODD group showed a significantly higher ODBI score at p < 0.0001. The concurrent use of this scale with clinical examination is expected to increase the accuracy of the diagnosis of ODD.