The self-attraction of nanowires (NWs) would lead to NWs bunching up together when fabricated in high density and the short circuit of NW-based devices during service. However, the underlying mechanism of the self-attraction of NWs remains debatable due to the lack of in situ characterization of the attraction. In this study, a versatile method of in situ investigating the self-attraction of NWs was developed. The attractive force between two NWs and their distance can be determined quantitatively in the process of attraction under an optical microscope, eliminating the influence of electron beam in electron microscopes. With this approach, the self-attraction of SiC NWs was investigated and a two-stage mechanism for the self-attraction was proposed. The electrostatic force between two individual SiC NWs increased as their distance decreased, and acted as the initial driving force for the attraction of NWs. SiC NWs remained in contact under van der Waals force until they separated when external force exceeded van der Waals force. The charge density and the Hamaker constant of SiC NWs were determined to be 1.9 × 10⁻⁴ C·m⁻² and 1.56 × 10⁻¹⁹ J, which played an important role in the attraction of NWs. The results shed light on the mechanism of self-attraction among NWs and provide new insights into fabricating high-quality NWs and developing high-performance NW-based devices.