The current study was undertaken to evaluate the effect of continuous force of different magnitudes (150, 50g) on alkaline phosphatase activity (ALP), with a focus on the amount of tooth movement after application of these forces. Eight female subjects (age range 13 to 15 years) participated in the 6 weeks study. A continuous retraction force of 150g was applied to 1 of the maxillary canines (group I) whereas a continuous force of 50g was applied to the other (group II) with a nickel-titanium closed coil spring. Gingival crevicular fluid (GCF) was collected from the mesial and distal sides of each experimental canine immediately before appliance activation and weekly over the following 6 weeks. The clinical gingival condition was evaluated at the baseline and at the end of the experiment. GCF ALP activity was determined spectrophotometrically at 37 C by kinetic method. The results showed insignificant changes in the clinical condition. GCF ALP activity was significantly greater in tension sites of group I and II as compared with the baseline. Moreover the enzymatic activity in tension sites of group I was significantly greater than that of group II. In both groups, a significant greater ALP activity was observed in tension sites compared with compression sites. The results showed insignificant difference in the amount of distal movement of the canines produced by the 2 forces. It was concluded that effective tooth movement can be produced with lower forces and ALP activity in GCF can be considered a suitable indicator of the biologic activity in the periodontium during orthodontic tooth movement.