Background: Mental illness and cognitive deviance can be more difficult to diagnose in children and adolescent than in adults due to high co–morbidity and varied clinical presentation and studies which deal with Rorschach Perceptual Thinking Index, providing data about its satisfactory diagnostic performances particularly in children and adolescents are few. Aims: To see the diagnostic efficacy of Rorschach Perceptual Thought Index (PTI) in assessment of thought disorder in children and adolescents with affective psychosis. Method: A total of 30 patients with clinical diagnosis of bipolar manic with psychosis, and manic with psychosis and 30 normal controls between 8 to 17 years was taken and Rorschach was administered. Results: The two groups differed significantly on the Rorschach PTI and will function most efficiently with a cut off score > 3 with high sensitivity, specificity, negative predictive power, positive predictive power, hit rate, low false positive rate & false negative rate. Conclusion: The Rorschach PTI appears to be an efficient measure of thought disorder in children and adolescents.