The monochromator for reducing energy spread of a primary charged particle beam in charged particle apparatus comprises a beam adjustment element, two Wien-filter type dispersion units and an energy-limit aperture. In the monochromator, a dual proportional-symmetry in deflection dispersion and fundamental trajectory along a straight optical axis is formed, which not only fundamentally avoids incurring off-axis aberrations that actually can not be compensated but also ensures the exit beam have a virtual crossover which is stigmatic, dispersion-free and inside the monochromator. The present invention also provides two ways to build a monochromator into a SEM, in which one is to locate a monochromator between the electron source and the condenser, and another is to locate a monochromator between the beam-limit aperture and the objective. The former provides an additional energy-angle depending filtering, and obtains a smaller effective energy spread.