Introduction: According to the literature, the use of lip bumper results in the relief of dental crowding. The mechanism underlying this process in unclear. Studying the treatment effects of LB three dimensionally may shade some light into the decrowding cascade. Methods: twenty female patients in the late mixed dentition stage with mild to moderate crowding in the mandibular arch were randomly assigned for either treatment or control group. The treatment group had continuous mandibular lip bumper treatment for six months. Study models were used to evaluate the changes in the arch dimensions and CBCTs were used to evaluate the changes in the incisors and molars angulations and to evaluate the bony changes. Results: It was found that lip bumper produced significant increases in all arch dimensions. The increase of arch dimensions is due to labial tipping of the incisors and distal tipping of the molars, in addition to a significant increase in the basal bony width. Conclusions: Lip bumper treatment in the late mixed dentition stage can significantly expand the mandibular arch dentally and skeletally.