Background: Mortality of pregnant mother is one of the most important indices for evaluation of socioeconomic development of a society and can show efficiency of health system of a country. Many causes of maternal mortality can be prevented or avoided. This study is conducted to specify the causes of maternal mortality. Materials and methods: This study is a descriptive-analytical research which has used retrospective method to investigate 165 records of maternal mortality in Legal Medicine Organization of Tehran during 2008-2013 and to provide the required information based on a researcher-made checklist from these records. Results: According to autopsy results, heart diseases with 34 cases (20.1%) have been the most important causes of mortality of pregnant mothers and after that, respiratory failures with 24 cases (14.5%) and obstetric complications (bleeding and infection after delivery) with 22 cases (13.3%) have been the most important causes of maternal mortality. Conclusion: Heart diseases, respiratory problems and bleeding have been the most important causes of maternal mortality and the importance of preventive measures for such health problems can be felt more than before. For the women with planning for pregnancy, the clinics should complete assessment of the cardiovascular and respiratory diseases fully and accurately in pre-pregnancy cares. Also, risk factors such as obesity, unhealthy diet and insufficient physical activity should be defined for them and required measures should be taken to meet these factors.