In this paper we develop a software reliability assessment tool, called PISRAT: Proportional Intensity-based Software Reliability Assessment Tool, by using several testing metrics data as well as software fault data observed in the testing phase. The fundamental idea is to use the proportional intensity-based software reliability models proposed by the same authors. PISRAT is written in Java language with 54 classes and 8.0 KLOC, where JDK1.5.0 9 and JFreeChart are used as the development kit and the chart library, respectively. This tool can support (i) the parameter estimation of software reliability models via the method of maximum likelihood, (ii) the goodness-of-fit test under several optimization criteria, (iii) the assessment of quantitative software reliability and prediction performance. To our best knowledge, PISRAT is the first freeware for dynamic software reliability modeling and measurement with timedependent testing metrics. Keywords: software reliability, reliability modeling and measurement, tool, proportional intensity based models, testing metrics.