Abstract: Fault tolerant development, proposed as an efficient approach of improving software reliability, is only applied to some safety critical systems due to its difficulties in design diversity and great increase in cost overheads. To deal with the situation, an applied framework based on a fuzzy reusable library is put forward. It adopts an enhanced reliability model to guide the development of fault tolerant software, which achieves diversity design through choosing reusable components with the same function on the basis of reliability parameters. The major advantages lie in simplifying the software development, and thus offer the improvement of system reliability in methodology and practice. The design approach is presented, which includes the architecture of fuzzy reusable library, enhanced reliability methodology, reuse based diversity design and their supports for fault tolerant development.