The Highway Capacity Manual (referred to as the HCM hereafter) 2000 model estimates roundabout entry capacity under an assumption of circulating headways following an exponential distribution. However, some studies indicated that the HCM 2000 model could over- or under-estimate entry capacity. This situation might be attributed to an unfit assumption of headway distribution type. Therefore, this study begins with analysis of headway distributions based on field survey. This study firstly indicates that the inverse Gaussian distribution is the best fitted distribution type of headway samples. According to this finding, we thus intend to adjust the HCM 2000 model. To this end, we propose a simulation based approach to estimate entry capacity of single-lane roundabouts. A linear relationship is established to represent the relationship between a capacity ratio of the simulation-based approach to the HCM 2000 model and conflicting flow. The linear function can be considered as a coefficient of the HCM 2000 model. It is believed that the modified HCM 2000 model outperforms the HCM 2000 model.