Abstract: This paper introduces a functional fault model for digital systems described by Register Transfer Language (RTL). A formal definition of standard statements in RTL is given, the functional faults are thug classified into eight types. A procedure for generating tests for any given functional fault is presented by using the inverse operation of the ETL description of the system under test. It is interesting to know that once the behavior of a digital system is described by RTL, similar ideas used in test generation for stuck-at faults can be pushed up to RTL level with each RTL statement as a "Component" of the system.