Software development managers and users are much interested in system performance such as not only software reliability but software availability. We develop a software availability model considering that the debugging activity becomes more difficult with the progress in fault-removal process. The time-dependent bahavior of the software system that alternately repeats the operational and maintenance states is described by a Markov process. Useful expressions for several quantitative measures of software system performance are derived from this model. Finally, numerical examples are presented for illustration of software availability measurement and assessment.