The aim of this study was to compare between the symmetry of upper dental arch in unilateral and bilateral posterior crossbite in early mixed dentition. Thirty children with posterior crossbite in early mixed dentition were incorporated in this study; 18 patients with unilateral posterior crossbite, and 12 patients with bilateral posterior crossbite. Impressions were taken before orthodontic treatment. The control group consisted of 30 casts of children in the same age group with accepted normal occlusion. To assess the dental arches, two lines were recorded on the cast; Medial line and Transversal line perpendicular to each other. In the sagittal plane, perpendicular distances of reference points from the transverse line were measured. In the transverse plane perpendicular distances of reference points from the median line were measured. The results indicated that, in the unilateral posterior crossbite group, there exist clinically significant asymmetries in 33% of patients, i.e. 67% show asymmetry less than 2 mm. In bilateral posterior crossbite, the distal segment seems predominantly symmetrical while prominent asymmetries were found in anterior segment. The control group with relatively regular dental arch shows a certain degree of asymmetry which does not manifest itself clinically, significant asymmetries in anterior and posterior segment were not found. This study concluded that posterior crossbites in unilateral and bilateral cases are mainly due to symmetrical contraction of the upper dental arches rather than dental arches asymmetry.