A new microfabrication technique that allows the precise construction of large three dimensional structures with dimensional tolerances approaching 1 micron is being applied to the design of a miniature scanning electron microscope (MSEM). In this paper we present the electron optic calculations of the MSEM source (gun). The MSEM measures less than one cubic centimeter and the source measures approximately 1 × 1 × 0.20 cm3. The details of the MSEM fabrication are in an accompanying article. There are many advantages of a MSEM. The performance of an SEM is improved as its length is reduced. The need for mechanical adjustments and motion feedthroughs is eliminated since the microscope components are pre-aligned to the optic axis. All components are ultra high vacuum compatible and can be heated to 500°C. A small, portable electron microscope can be brought to the sample to be inspected instead of the sample being brought to the microscope. Vacuum hardware requirements are minimized. The fabrication technology is inexpensive with respect to the conventional methods of electron microscope construction. Arrays of MSEMs can be built to allow applications in high throughput e-beam lithography. In addition two MSEMs can be mounted a few degrees apart to provide stereo imaging.