A head circumference adjustment device of a helmet includes: a main body formed with a cavity; an annular strap having a first end and a second end passing through the cavity, each of the first and second ends being formed with a rack section, a thumb section and a shoulder section; and a controller mounted in the cavity. The controller has a gear engaged with the rack sections of the strap. By means of rotating the controller, the position of the strap can be adjusted to adjust the head circumference. The head circumference adjustment device can be easily operated and truly fixed to overcome the shortcomings existing in the conventional adjustment device that the strap is likely to slip and the position of the strap can be hardly truly adjusted and the strap is likely to detach from the adjustment device.