Death is a term that means ending of life, or the end of all life functions of organism. Thanatology is part of forensic medicine which studies death. The cause of death is a pathological condition that leads to the death of a person while the mechanism of death describes disorder that is direct consequence of the cause of death itself. When determining the cause of death, forensic autopsy has very important role, but in some cases, despite all available and applied methods and efforts, the cause of death remains unknown. At the Department of Forensic Medicine and Criminology in Rijeka amount of unknown and undetermined causes of death for a period between 2000. and 2014. is 0.7 % which is in good correlation with percentage from Report of death in Croatia. The largest number of undetermined causes of death were skeletonised remains and bodies in advance stadium of putrefaction. Results from the studied group showed us that unknown cause of death is more frequent between older age groups and mostly between male decedents. Frequency of unknown and undetermined deaths represents good indicator of the overall mortality statistics.