Single-layer δ-As and γ-P have unique atomic arrangement, which belong to the Pmc21 and Pbcm space groups, respectively. Because of the coupling hinge structure, the physical properties of the two materials have obvious anisotropy. In this paper, we report the mechanical properties of the single-layer δ-As and γ-P. That is, their inherent negative Poisson ratio (NPR) is -0.708 and -0.226, respectively. Surprisingly, the absolute value of the NPR of δ-As is approximately 26.2 times greater than that of single-layer black phosphorus (the NPR of single-layer black phosphorus is -0.027), and remains invariant at a certain strain range. Thus, single-layer δ-As will have huge potential applications in nanosensors and electronic wearable devices due to its invariant and large, negative NPR.