Abstract: Objective: To assess the association between the developmental stages of mandibular third molars and the space distal to the permanent molars (retromolar space), and to predict the growth potential of retromolar space and provide diagnostic basis for clinical treatment. Methods: The materials included pretreatment lateral cephalographs of 129 orthodontic patients (62 males, 67 females; 8-18 years old) from Urumqi Stomatological Hospital. The molar formation stage was evaluated by the method of Nolla. The retromolar space was measured from the most concave point of the anterior border of the ramus to the distal surface of the first molar by CBCT. Statistical analyses included t-tests, the Pearson product moment and the linear regression equations. Results: Between gender groups the retromolar space was different, and the age and formation stages of the mandibular third molars had no statistical difference. The correlation between retromolar space and developmental stage was high (r=0.860,P<0.001). Conclusion: We can predict the mandibular retromolar space by estimating the dental age of mandibular third molars, and provide reference for clinical diagnosis and treatment planning