Background: The aim of the study was to examine the ethnic differences in knowledge, attitude and practice towards mental illness in a sample of Qatari and non-Qatari Arabs. Subjects and Methods: This is a cross sectional survey conducted in Primary Health Care centers, Qatar from October to June 2009. A representative sample of 3000 Qatari and non-Qatari Arabs above 20 years of age were approached and 2514 subjects (83.8%) gave consent to participate in this study. Results: More than non-Qatari Arabs, a significant proportion of Qataris thought that mental illness can be a punishment from God (44.5% vs 50.6%; p=0.002) and that people with mental illness are mentally retarded (35.1% vs 45.1%; p<0.001). Qatari nationals had a poor knowledge about causes of mental illness compared to non-Qatari Arabs such as a belief that mental illness is due to possession of evil spirits (40.5% vs 37.6%) and psychiatric medication will cause addiction (61% vs 57.3%). Conclusion: The study revealed that there is an ethnic diversity within Arab societies in their knowledge, attitude and practice towards mental illness.