Cytokines are expressed by cells within the periodontium in response to mechanical stress produced by orthodontic appliances. These cytokines can modulate process of bone remoldeling during orthodontic tooth movement. The gingival crevicular fluid (GCF) composition is supposed to reflect the physiological status of the periodontal ligament. In order to monitor the expression of cytokines in humans non-invasively, changes of the compositions of gingival crevicular fluid have been studied. The data of recent study showed that the application of orthodontic force induces synthesis of cytokines which support the hypothesis that these cytokines play important roles in bone resorption during the application of orthodontic force.