New dimensional NiCo alloy icosahedral nano-crystals with controllable size have been first reported and synthesized through an Ostwald ripening process in a template-absent solvothermal reaction system. The proposed synthesis is corroborated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction analysis (XRD), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). The as-obtained NiCo icosahedral nanocrystals exhibit the size- and component-dependent magnetic behaviors. The coercivity (H-c) depends on both the magnetocrystalline and structure anisotropy, and the saturation magnetizations (M-s) decided by the content of Co. H-c decreases from 189.02 to 147.95 Oe with the increase of the icosahedral NCs size from 200 to 850 nm. Especially, the H-c of the icosahedral NCs at 157.38 Oe is higher than that of nanospheres at 104.02 Oe. In addition, M-s and H-c increased with the increasing Co content. It can be an ideal building block for applications in magnetic media, sensors, and other devices.