Background and objectives：Traditional Japanese sweets are very popular and are often eaten in middleage and elderly people. A lot of traditional Japanese sweets are made from adzuki bean paste, which is called “An”. Adzuki beans are a good source of carbohydrate as well as of protein, because they are starchy pulse. Resistant starch（RS）escapes digestion until reaching colon and acts like dietary fiber. Recently, many researchers suggest taking this new type of dietary fiber for our health benefits. The purpose of this study was to investigate that the effect of different cooking times on RS contents and particle morphology of adzuki bean paste “An”.Methods：Adzuki beans were boiled 50, 70, 90 minutes with five times volume of water. After grinding, the mixture was strained through a sieve in order to remove husk and put into cheesecloth. Then, 6kg of stone was placed on the cheesecloth for 1 hour to dehydrate “An”. Each treatment of “An” was analyzed RS contents and observed “An” particles by optical microscope.Results：RS contents of “An” of 50, 70, 90 minutes cooking time were 6.4%, 5.0%, 4.4%, respectively. These results showed that the longer adzuki beans were cooked, the less amounts of RS were formed. Optical microscope observation showed that longer cooking time increased damaged or ruptured “An” particles.Conclusions：These results indicated that damaged “An” particles had more digestible than intact “An”particles. It might be possible that intact “An” particles resist digestive enzymes. Furthermore, starch inside ‘An’ particles might be altered their structure during cooking. These change would be affected RS contents of adzuki bean paste “An”.