Background: Cathepsin K and alkaline phosphatase have been found to be significantly elevated in teeth undergoing orthodontic forces compared with untreated controls. The present study assessed the level of Cathepsin K in GCF during orthodontic tooth movement. Materials & Methods: This study was conducted on 12 patients age ranged 12-22 years of both genders needing orthodontic treatment. for Angle’s Class I bimaxillary protrusion. From each patient 4 GCF samples were collected 1 hour before treatment, after 1 day, 7th day and 1 month after the placement of the orthodontic appliance on maxillary left canines and right canines. GCF of approximately 2 microliter was collected from the distal sulcus of the canine by extracrevicular method using a graduated micro capillary pipette. cathepsin K in GCF was estimated using ELISA method. Results: Out of 12 patients, there were 5 males and 7 females. The mean Cathepsin K level at 0 hour in test side was 142.5 and in control side was 140.2, at 1 day was 52.6 and at control side was 51.8, at 7 days was 341.2 and at control side was 280.4, at 30 days was 48.2 and at control side was 92.4. The difference was significant (P<0.05). Conclusion: Authors found increase level of Cathepsin K on test side as compared to control side on 7th day. The amount of Cathepsin K in GCF increased during the initial period of orthodontic tooth movement. Key words: Cathepsin K, ELISA, Gingival crevicular fluid