The form of determination of ideal arch has been attempted by many authors, without reaching only one form, as variations exist in the same individual for sex, race and environmental factors. A cross sectional study was carried out in a non-probabilistic sample of 40 Mapuche children and 35 Non-Mapuche children in which the maxillary and mandibular form was determined through morphometric ruler with forms of triangular, ovoid and square arch (Orthoform I, II and III). In the analysis of arch forms for every group and its association, some differences were observed in the square and triangular distribution of the forms, these are however not statistically significant. In reference to the coincidence of the arcades was for the Mapuche Kappa Index of 0.3096 and for Non-Mapuche of 0.1204 indicating that concordance does not exist in both groups. There are differences in the form of arch between Mapuche and Non-Mapuche. The predominant form in both groups is the ovoid form. There is a lesser coincidence of maxillary and mandibular form to jaw arch in the same individual.