Introduction: Rapid maxillary expansion is mostly used in the treatment of narrow maxilla by splitting the mid-palatal suture. Rapid maxillary expansion appliance also causes anterior displacement of maxilla. This study was carried out to evaluate the volumetric changes in paranasal sinus after maxillary expansion therapy in growing patients. Materials and methods: Pre (Group T1) and post expansion (Group T2) CBCTs of 15 subjects were considered to evaluate the changes in the dimensions of maxillary sinus. The data obtained was found to be non-normally distributed. So, non-parametric test named Wilcoxn signed-rank test was used. Interclass correlation coefficient between subjects in T1 and T2 was done. The results showed a significant excellent positive correlation between subjects with r value of 0.998 and p value <0.05.Intra-class Correlation of reliability was above 0.8 for all variables. This proves agreement between analysis done at two time frames T1 and T2 which obviates intra-observer error of measurement. Results: On comparison of the mean values of maxillary sinus pre (T1) mm3 and post (T2) mm3, the mean value of maxillary sinus post expansion is higher with a p value of 0.001which denotes statistically significant changes. Conclusion: The rapid maxillary expansion appliances play a significant role in the improvement of maxillary sinus volume, thus decreasing the lower airway resistance. This proves to be very beneficial in mouth breathing patients. Keywords: Maxillary sinus volume, Rapid Maxillary Expansion, Volumetric changes, CBCT, HYRAX