The role of inherent Ca2+ in phosphorus removal from wastewater was evaluated by batch tests. Precipitates were characterized by an X-ray diffractometer (XRD), Fourier transform infrared spectrophotometer (FT-IR) and scanning electron microscope with an energy dispersive spectrometer (EDS) system. Effects of inherent Ca2+ on phosphorus removal through basic oxygen furnace slag (BOFS) were also analyzed. The results show that upon adjusting the pH to higher than 7.0, inherent Ca2+ can remove phosphorus from wastewater and form Ca-P precipitates. Residual phosphorus exhibited a linear decreasing trend with increasing the pH from 7.0 to 10.0 and then remained unchanged at higher pH than 10.0. EDS determined that the precipitates contained the elements Ca, P and O. FT-IR spectra demonstrated that the functional groups of precipitates involved PO43−, OH− and CO32−. XRD indicated that the precipitates may consist of CaCO3 and some Ca-P phosphates such as CaHPO4, Ca4H(PO4)3, Ca3(PO4)2, and Ca5(PO4)3(OH). During the removal process of phosphorus by BOFS, due to the presence of inherent Ca2+ in wastewater, the removal efficiency and rate of phosphorus increased by 15.5% and by a factor of about 3.0, respectively.