The abacus is a calculating tool that has been used in Asia for thousands of years. Mental abacus calculation is a skill in which an abacus image in the mind is used without the actual physical manipulation of the abacus. Using this method, people can perform extremely rapid and accurate mental calculations. Research indicates that abacus training can help develop concepts of number, place value, operation sense, and mental computation. In this study, the effects of mental abacus training on the mathematics achievement of children with mild/moderate mental retardation were examined. Rather than being taught as an isolated subject, abacus operation and mental abacus calculation were imbedded into all aspects of a math curriculum. The broad domain of children's mathematical knowledge including number concepts, computational skills, application, and problem solving skills were investigated. (Contains 1 table and 1 figure.)