Providing accurate estimation during project planning is crucial to meet the time lines of the project, however, with all the emphasis we put when estimating the activities in a project, we still have difficulties in predicting realistic estimation for some activities, especially the activities that have never experienced before. The problem here is that there are hidden variables that are difficult to anticipate, and sometimes cost a lot of time to fix them. Many techniques are available to help project managers successfully providing realistic estimation, one of these techniques advices to maintain the historical data for the work, during the planning process, if details of the same activities are available, then it will help the project manager to think about all variables related to an activity. Ontology has the potential to play an important role in classifying, storing and querying information. In this paper we suggest a solution that uses ontologies to maintain the historical data for the work, and it allows sharing the data between multiple firms.