This study was carried out to investigate the effect of relative age on anthropometric properties and motor performance in Turkish children (girls n=423, boys n=601). Anthropometric measurement sites and techniques have been set out by the ISAK (International Society for the Advancement of Kinanthropometry). A group of tests involved in Eurofit Test Battery and other standard tests were used. For each age, the data of those who were born within the first three months and the last three months of the year were compared. The MedCalc Statistics Program was used for the differentiation and variation percentages between two periods were studied (p=0.001, p=0.05). Consequently effect of relative age was observed on anthropometric characteristics and motor performances of Turkish girls and boys between 8 and 12 years old. Researchers, trainers, families, sports managers and organizers are advised to consider Effect of Relative Age.