Introduction: Orthodontists can benefit from understanding occlusal changes during every stage of human development. The growth and development period is influenced by environmental factors, nutrition, and ethnic variations; systemic health and individual variations can also occur. The aim in this study was to study the changes in the dental arch dimensions with age from mixed dentition to early permanent dentition in a group of Yemeni school pupils. Methods: One hundred and Twenty subjects (60 females, 60 males) divided into three groups Gp1 (8- less than 10 yrs), GP2 (10 - less than 12 yrs) and GP3 (12 -14 yrs). Upper & lower study casts were evaluated in centric occlusion. The casts were scanned and 9 parametrs; arch perimeter, arch length, inter-canine width, inter-molar width, and palatal depth; were measured on the computed cast images by using special software. In addition 2 parameters; overjet and overbite; were measured manually. Results: The maxillary arch perimeter showed gender effect at GP3; however, the other parameters there were no gender effect. There were significant changes in arch perimeters, arch length, overbite, and palatal depth in Yemeni school pupils  between the mixed and the permanent dentitions. Conclusions: These results should be useful in planning orthodontic treatmen for Yemeni patients in the mixed and early permanent dentition.