A scan mechanism and a scan module are provided. The scan mechanism adapted to be used in a scan module includes a rack and a scan unit. The rack has an engaging surface, a bottom surface and a plurality of teeth disposed on the engaging surface. Each of the teeth comprises a top end, wherein the distance between the centers of any two adjacent top ends is W, the shortest distance from the top end of each tooth to the bottom surface of the rack is D, and W/D is from 0.56 to 0.79. The scan unit includes a gear engaged with the teeth of the rack and adapted to roll along the engaging surface of the rack, wherein the scan unit is slidably disposed on the rack through the gear.