OBJECTIVE To determine the most common maxillary and mandibular arch form and size in primary dentition stage. METHODS Four hundred thirty five preschool children were included in this study with an age range of 3.5-5.5 years old. Dental casts were digitized and landmarks for the measurements were identified and marked on each model. Ten points were determined on the dental arch. Measurements of the x and y coordinates of the 10 points of each dental cast image were plotted on a computer software to obtain the polynomial function that best describes the curve corresponding to the dental arch form. RESULTS Three arch forms were observed; square, oval and tapered. The most common arch form observed in the primary dentition was the oval form (52%) followed by the square and tapered arch forms (29% and 19%, respectively). Gender differences were not detected. In the maxilla, the most common arch form was the oval (58.4%) arch whereas the square arch form (46.7%) was the most common in the mandible. The medium size arch form was the most common size in both mandible (83%) and maxilla (71.5%) and for the total sample (77%). Differences in arch form size distribution were detected (P<0.000). Small size arch form was more common in females compared to males in the maxilla and total sample (P<0.000). Intercanine width was significantly smaller in the tapered arch form compared to oval arch form (P=0.048). In both arches, intercanine and intermolar widths were significantly larger in males compared to females. CONCLUSIONS The most common arch forms in the maxilla and mandible were the medium sized oval and square forms, respectively. Intercanine width was the smallest in the tapered arch form. Intercanine and intermolar widths were affected by gender.