Phosphorene is another layered elemental structure after graphene. The mechanical property is an important character for two-dimensional (2D) structures. The 2D Young’s modulus (E^2D), representing the deformation resistance of 2D nanostructures, has been investigated for the black phosphorene (BP) nanoflakes with thicknesses of 6–22 nm (11–43 layers) by an atomic force microscope nanoindentation method. The nanoeffects have also been found to significantly affect the E^2D of BP nanoflakes. The BP E^2D increases as the layers decreasing. The E^2D-layer model for BP nanoflakes has been deduced to follow E^2D(n) = 53.31(1 + 3.30e^–0.13n), where the E^2D of monolayer BP was deduced to be 214 ± 11 N/m. The E^2D analysis model and E^2D-layer model have been well used to deduce the E^2D of BP nanoflakes.