An adjusting mechanism and a head mounted display are provided. The adjusting mechanism includes a band, a rotating shaft, a knob, a driving member, and a holder. The rotating shaft has a first ring tooth around a central axis. The band is driven by the rotating shaft to move relative to the rotating shaft when the rotating shaft rotates around the central axis. The knob has a plurality of chutes. Each chute has a first section and a second section. The depth of each first section is deeper than the depth of each second section. The driving member has a second ring tooth, a plurality of guiding pins, and a plurality of pawls. The driving member is assembled to the knob, and the guiding pins are located in the chutes. The holder has a circular unidirectional tooth. The band limits the rotation of the holder relative to the band. The circular unidirectional tooth is configured to be coupled with these pawls to limit the rotation of the driving member relative to the holder in a single direction.