The ship collision is one of the most significant casualties occurring at sea. Some research on maritime casualty reveals that over 90% of all collisions were attributable to human element. Although frequently occurring unsafe acts in collisions have been roughly identified, very little is known about the human element and the reasons why mistakes occur repeatedly. The main objective of this paper intends to identify the human elements especially the underlying human elements and influencing factors behind the unsafe acts involved in collisions at sea. For this purpose, 100 existing written accident reports are selected and used as the data sources for human element analysis. As compensation, an investigation on the viewpoints of maritime safety related persons has been carried out by questionnaires. A classification for collecting and storing the data of human elements involved in collisions is specially constructed. It seems that Expert's perspectives on human elements involved in collision accident are roughly identical with the actuality data derived from the written reports. It is found that the most frequently involved unsafe act in collision is poor lookout, the underlying human elements are lack of experience and knowledge, and the external factors are fairway