In the present study, a comprehensive description and a complete process for designing a high‐resolution low‐voltage scanning electron microscope with a field emission gun have been discussed, including the design of two types of magnetic immersion lenses optimized for low‐voltage operation, the evaluation of their resolution, the design of the magnetic lens/secondary electron collector system, the calculation of three‐dimensional trajectories for both secondary electrons and backscattered electrons, and the estimate of their collection efficiency. The computed results and the primary experiments indicate the possibility of achieving nanometer resolution, which basically approaches the electron optical predictable probe size for the low‐voltage scanning electron microscope.