Abstract-Today's Grids include resources (referred as Grid-site s) from different domains including dedicated production resources, resources from university labs, and even P2P environments. Grid high level services, like schedulers, resource managers, etc. need to know the reliability of the available Grid-sites to select the most suitable from them. Modeling reliability of a Grid-site for successful execution of a job requires prediction of Grid-site availability for the given duration of job execution as well as possibility of successful execution of the job. Predicting Grid-site availability is complex due to different availability patterns, resource sharing policies implemented by resource owners, nature of domain the resource belongs to (e.g. P2P etc.), and its maintenance etc. To give a solution, we model reliability of Grid-site in terms of prediction of its availability and possibility of job success. Our availability predictions incorporate past patterns of the Grid-site availability using pattern recognition methods. To estimate possibility of job success, we consider historical traces of job execution. The experiments conducted on a trace of real Grid demonstrate the effectiveness of our approach for ranking Grid-sites based on their reliability for executing jobs successfully.