One of the definitions for testability is the probability whether tests will detect a fault, given that a fault in the program exists. The testability can be estimated from the probability of each statement fault leading to output failure. The probability of the test detecting a fault depends on the probability of individual statement faults appearing as an output failure when a fault exists at a statement. The testability measure of the software has been introduced based on output failure probability and the entropy of the importance of basic statements to the output failure from the software fault tree analysis. The output failure probability and the importance of statements are calculated from software fault tree analysis. The Suggested testability measure has been applied to the two modules of the safety system in a nuclear power plant. The proposed testability measure can be used for the selection of output variables or to determine the modules that are more vulnerable to undetected faults. (C) 2003 Elsevier Inc. All rights reserved.