We collected bug tracking data from a few popular open source projects and investigated the time related bug reporting patterns from them. The results indicate that along its development cycle, open source projects exhibit similar reliability growth pattern with that of closed source project. Bug arrivals of most open source project will stabilize at a very low level, even though in comparison, no formal testing activities are involved. This stabilizing point would be viewed as the mature point for adoption consideration. The results also show that general Weibull distribution offers possible way to establish the reliability model; Also, popular measures such as page views and download are not highly correlated with the bug arrival rate and may not be suitable measures for a project’s quality.