A baseball batter's helmet includes a shell; a protective padding including crown and arcuate pads; an adjustment assembly including a split ring including two elongate end openings, a first toothed section formed on a top edge of one opening, a second toothed section formed on a bottom edge of the other opening, a rear plate including a recess having a toothed member and a transverse channel with the openings inserted thereinto to form a crossing relationship, a disk including a gear meshed with the toothed sections, and two opposite hooks engaged with the toothed member, and a rotatory knob mounted on the shell and secured to the disk, the split ring, and the rear plate; and a knob protector. Clockwise turning the knob to cause the openings to move toward each other by activating the gear mechanism will tighten the crown pad to decrease a diameter of the arcuate pad.