Abstract: An improvement in the structure of a monitor filter includes a generally rectangular primary frame having an inverted L-shaped cross section, four brackets at four corners each provided with a spring-loaded ball, a pair of secondary frames slidably engaged with two sides of the primary frame, each of the secondary frames being provided with two lugs each having a horizontal slot formed with a plurality of teeth engageable with the ball, and an anti-radiation screen mounted on the primary frame by a rectangular member, whereby the secondary frames can slide in or out of the primary frame to adapt for use with monitors of different sizes.