Objective: To study the effect of force decay on the processes of resorption and repair of root surface. Methods and Materials: Twenty one prospective patients requiring the extraction of mandibular first premolars were recruited according to strict selection criteria. The participants were assigned to three groups according to the period of force application – a five-, seven- and nine-week group. The first premolar on one side was selected at random. An experimental orthodontic appliance was used to deliver 75 cN of buccal tipping intrusive force using NiTi coil spring. The force was not reactivated. At the end of the experiment, the first premolars were extracted. Seven randomly selected non-moved contralateral premolars served as control. The test and control premolars were examined using scanning electron microscope. Results: Significantly less surface resorption was registered in the five-week test group. The seven-week and nine-week test group showed no statistically significant difference in surface resorption. Apical resorption scores were strongly associated with the experimental periods. Only partial repair of resorption lacunae was evident in the seven- and nine-week group, involving both cellular and acellular cementum. Seven weeks appeared to be sufficient for the process of root resorption to cease, however a nine-week period did not seem long enough for reparative cementum to overlay the resorption lacunae. Conclusion: Using a similar force system, an initial force less than 75 cN or an activation cycle longer than 9 weeks would allow for more repair of resorption lacunae.