Based on observed data, this paper evaluates the performance of capacity estimation for single-lane roundabouts using the Highway Capacity Manual (HCM) 2000 model. In this study, the HCM 2000 model is indicated to be over- or under estimate roundabout entry capacity. This is because the HCM 2000 model estimates capacity using an unfit assumption of headway distribution type. According to this finding, we propose a modified model to adjust the HCM 2000 model. Based on a comparison study, the modified model produces smaller relative error (0.92) and root-mean-square deviation (12.79) than the HCM 2000 model (4.66 and 48.17, respectively). It is believe that the modified HCM 2000 model outperforms the HCM 2000 model.