Root resorption may be one of acute problems that need to be solved in orthodontic treatment. Many causative factors have been reported, and the orthodontic force magnitude may be one of the most dangerous risk factors. Upper rat molars have been frequently used to analyze tooth movement and root resorption induced by orthodontic force. Three-dimensional tooth movement in the rat model will be shown, and the relationship between the tooth movement, force magnitude and root resorption will be discussed. Furthermore, the effect of anti-inflammatory drugs and fluoride on root resorption will be shown, and the possibility of preventive method against root resorption will be discussed.