The friction of orthodontic appliances is recognized to be detrimental to tooth movement. The purpose of this study was to determine the magnitude of frictional force changes between bracket(018″X025″ solt) and orthodontic wires(stainless steel, cobalt-chromium. and β-titanium, 017″X0.25″ rectangular) with time. The wire was secured in the bracket slot with a elastomeric ligature. Frictional forces were measured by universal testing machine. The following conclusions were obtained. 1. The frictional forces under dry condition were greater than those in saliva. 2. The frictional forces produced by cobalt-chromium wire were less than those generated by stainless steel and β-titanium wire. 3. The frictional forces increased progressively with time, and the amount of increase on first two weeks was greater than on last two weeks. 4. The change of frictional force under dry condition was greater than in artificial saliva.