This study based on self-determination theory aimed to assess the relationship between motivation, competence in diabetes management and perceived autonomy support respectively and mental illness self-management; furthermore, the authors intended to explore the determinants of mental illness self-management for patients with schizophrenia and diabetes. This was a cross-sectional study. One hundred ten participants were recruited at the psychiatric hospital and assessed with the Illness Management and Recovery Scale, the Treatment Self-Regulation Questionnaire, the Perceived Competence Scale for Diabetes and the Health-Care Climate Questionnaire. Patients with lower educational levels, who were admitted to the chronic ward, were unemployed, had lower motivation in illness management, lower competence in diabetes management and perceived lower autonomy support all had lower mental illness self-management. Competence in diabetes management, employment status, autonomy support, educational levels and living alone were the determinants of mental illness self-management. Recovery from schizophrenia and comorbid diabetes is an enduring and complicated process requiring support from healthcare providers. Health professionals should assess the levels of illness self-management and provide integrated care interventions for patients with schizophrenia and diabetes, helping them manage both conditions. The illness management programme could apply to patients with schizophrenia and diabetes and address patients' characteristics such as low levels of education, living alone and being unemployed while additionally promoting motivation and competence, and providing supporting autonomy. The STROBE checklist was followed. Patients with schizophrenia and diabetes were recruited for this study and voluntarily completed the questionnaire.