The relaxed structures and electromagnetism properties of Fe nanobelts with different cross-sections of 3×5, 3×7, 3×9, 3×11, 3×13, and 3×15 atom layers were investigated using the first-principles of projector-augmented wave (PAW) pseudo potential based on the density functional theory (DFT) framework. Results show that for all the Fe nanobelts, the relaxed structures retain the two-fold symmetry. However, the cross-section changes from rectangle shape in the beginning into a near-single-ellipse shape for the ones with atom layers of 3×5 and 3×7, and into a double-ellipse shape for other atom layers with broader cross-sections. In addition, it is found that the Fe nanobelt with 3×7 atom layer is a half-metal material: electrons with either majority spin or minority spin can pass through the Fermi level. Therefore, it can be used in the field of spintronics for producing nearly 100% spin-polarized currents.