An offshore trawler “Ryuho Maru No.5” capsized and foundered off the Erimo Cape of Hokkaido on 11 September 2000 in moderate sea condition. In this paper the factors related to the accident are evaluated. As the main factors followings are listed; 1. hauling much amount of fish on the upper deck, 2. rough manoeuvring, 3. movement of unlashed items on board, and 4. movement of the cod end with fish over the inner bulwark. Evaluating the effect of each factor on stability and on heeling moment, the sequence and mechanism of capsizing were clarified from the energy balance like the C-coefficient concept. Moreover the effects of dominant factors are investigated through a parametric study. It is clarified that the heavy cod end on deck and centrifugal force by hard turning are of prime importance because the former not only reduces the stability but leads to heeling moment, and the latter keeps working along the capsizing motion.