We studied the atomistic structure of boron atom at the Si(001)ZSiO 2 interface using ab initio calculation method to investigate the mechanism of boron pile-up at the interface. We found that, if there is no defects, such as oxygen vacancy, at the interface, no stable sites of B would appear at Si/SiO2 interface and SiO2 layer, thus indicating that boron in silicon will only diffuse to the interface, but not segregate across the interface, unless additional defects or impurities exist. By introducing oxygen vacancy and H bonds, we found some stable configurations at Si/SiO2 interface, which can support the mechanism of boron segregation at Si(001)/SiO2 interface. Therefore, we assume that vacancy of O and H bonds may play a crucial role in segregation by opening additional trapping sites. Furthermore, we also found the largest energy difference between B at Si/SiO2 interface and that in deep bulk Si is about 2.9eV, which is in agreement with experimental boron activation energy of emission from Si/SiO2 value of 2.64eV.