Background: The purpose of this study was to assess the dental arch dimensions in primary dentition of 3-5 years old Filipino children and determining the existence of sexual dimorphism.Materials and Methods: In this descriptive study, 60 Filipino preschool children aged 3-5 years old were selected from Manila city. Study casts were used for measurements of 3 arch dimensions: bicanine width, bimolar width, and arch length. The analyses were performed to compare values within age-groups and within gender-groups, besides reporting mean and standard deviations. The data were subjected to One-way ANOVA and Student t-test.Results: Significant differences were observed regarding dental arch dimensions among different ages and genders (p<0.05). Comparison between genders showed a statistically significant difference in bicanine width at the age of 4 (p=0.04), in bimolar width at age of 5 (p=0.006), and in arch length at age of 4 (p=0.03) and 5 (p=0.02). Differences within male group for different ages showed only arch length dimension was statistically significant (p=0.021). In females, however the comparison between age-groups also revealed a statistically significant difference in bicanine (p= 0.016) and bimolar widths (p=0.027) dimensions.Conclusion: The present study showed that dental arch dimensions, vary among different ages and genders of children. It also showed that a sexual dimorphism does exist between the gender groups.