Abstract: A novel fault tolerant primitive, called combinational transaction block （CTB）, is presented here. It is constructed based on the principles of N version programming, recovery block, and dynamic redundancy structure of hardware fault tolerance. Therefore, CTB can be considered as a combinational fault tolerant mechanism for algorithms, programs and data. Its implementation in C language and FOXPRO is discussed in detail. The executing time of CTB on redundant processor systems is analyzed. The availability of CTB is demonstrated by software fault tolerant experiments.