Nanostructured body-centered cubic (BCC) metals exhibit remarkable mechanical properties under various stress fields, making them promising candidates for novel micro/nanoelectromechanical systems (M/NEMS). A deep understanding of their mechanical behaviors, particularly at the atomic scale, is essential for optimizing their properties and expanding their applications at the nanoscale. Newly developed nanomechanical testing techniques within transmission electron microscopy (TEM) provide powerful tools for uncovering the atomic-scale microstructural evolution of nanostructured BCC materials under external forces. This article reviews recent progress in the experimental methods used in in situ TEM nanomechanical testing and the achievements of these techniques in understanding the deformation mechanisms of BCC nanomaterials. By outlining the current challenges and future research directions, this review aims to inspire continued exploration in the nanomechanics of BCC metals, contributing to the development of advanced BCC nanomaterials with tailored mechanical properties.