in order to increase computer system reliability, software fault tolerance methods have been adopted to some safety critical systems including nuclear power plants (NPPs). Prevention of software common mode failures is a crucial problem in software fault tolerance, but an effective method to solve this problem has not yet been found. Our research, to find an effective method to prevent software common mode failure s experimentally examined the impact of specification language diversity on NPP software diversity. Three specification languages were used to compose three requirements specifications, and programmers made 12 product codes from the specifications. From the product codes analysis using fault diversity criteria, we concluded that a diverse specification language method would enhance program diversity through diversification of requirements specification imperfections. (C) 2001 Elsevier Science Inc. All rights reserved.