This invention provides a design of Wien filter for satisfying Wien Condition so as to ensure the Wien filter's performance. At first, to minimize the magnetic flux leaking out of the Wien filter, the invention proposes three measures to form a magnetic circuit to cover the magnetic device of a Wien filter respectively. The measures especially benefit a Wien filter acting as beam separator or Monochromator in a high resolution SEM. Secondly, based on the Wien filter proposed in cross-reference, several ways are provided for reducing the dissatisfaction of Wien Condition within the Wien filter, which especially modify either or both of the distribution shapes of the on-axis electric and magnetic dipole fields at two ends of the Wien filter. These ways provide more flexibility to reduce the dissatisfaction of Wien Condition in a Wien filter to a given degree at a reasonable manufacturing cost.