Objective: To evaluate and compare the masseter muscle length, volume, and fiber orientation in a skeletal class II subject before and after a twin block appliance. Background: Orthodontic interventions play a pivotal role in the correction of malocclusions, particularly in growing individuals with Class II skeletal patterns. The Masseter muscle, a key component of the masticatory system, undergoes dynamic adaptations in response to functional appliances, influencing its length, volume, and fiber orientation. Among the various orthodontic appliances, the Twin Block has gained prominence for its effectiveness in addressing Class II discrepancies. These abstract aims to provide a comprehensive evaluation of the alterations in Masseter muscle characteristics in growing skeletal Class II subjects, both before and after the application of the Twin Block appliance. The Masseter muscle's intricate relationship with mandibular development and function underscores the significance of understanding its adaptive changes during orthodontic treatment. By employing advanced imaging techniques and quantitative assessments, we seek to elucidate the impact of Twin Block therapy on the length, volume, and fiber orientation of the Masseter muscle. A thorough exploration of these morphological changes can contribute valuable insights into the biomechanical aspects of functional appliances and their influence on craniofacial growth. The findings from this study may not only enhance our understanding of the muscular adaptations associated with Twin Block therapy but also provide clinicians with valuable information for optimizing treatment strategies in growing Class II patients. As we delve into the dynamic interplay between orthodontic appliances and muscular responses, we strive to advance the knowledge base in orthodontics and contribute to the refinement of evidence-based treatment protocols. Methodology : It was a prospective longitudinal study in the Department Of Orthodontics And Dentofacial Orthopaedics. The calculated sample size was 19 so rounded 20 ( 11-Female and 9male)patients were included in the study. The study incorporates information about the ultrasound (USG) of the Masseter muscle, highlighting its low cost and no radiation exposure to the patient in our study we have used Samsung [ RS 80 A] Ultrasonography machine, liner probe LA3- 16 A for measuring the various parameters like length, volume, and fiber orientation of Masseter muscle. The statistical analysis was done by SPSS 21 statistical software for windows 11.