External apical root resorption was a common phenomenon associated with orthodontic treatment. The reported incidence ranged from 0% to 100%. It was caused by several factors, which could be mainly divided into biological and mechanical aspects. Biological factors included genetic susceptibility, systemic diseases, gender and medication intake. Mechanical factors consisted of the magnitude of orthodontic force, force type and duration, types of tooth movement, as well as excessive tooth movement. This review discussed the relationship between the movement type during retracting the anterior teeth and the external apical root resorption and its evaluation methods.