Although many studies have evaluated the impacts of bioretention cell (BRC) design elements on hydrologic performance, few have investigated the roles played by site characteristics and rainfall patts. The objectives of this study were to assess the impacts of rainfall and catchments with different characteristics on the hydrologic performance of BRCs and identify important factors in sizing bioretention when hydrologic performance was oriented for the design using a modeling approach. A 10-year record of rainfall data was used to identify the frequency and magnitude of rainfall events. The results showed that although the small and medium rainfall events were dominant they contributed less to the total rainfall depth than the large rainfall events. The ratio of runoff coefficient to imperviousness can be used as an indicator to explain why BRCs perform differently with the same design strategy under the same rainfall events. Rainfall patts had significant impacts on the hydrologic performance of BRCs by influencing the overflow and underdrain flow. BRCs performed better for rainfall events with a longer duration and lower rainfall intensity because they generated smoother runoff processes into the BRCs. On the basis of these results, the runoff coefficient is suggested for BRC surface design.