Objective To investigate the effect of loop configuration on appliance stiffness of stainless steel round wire. Methods Some stainless steel round wires with various cross-sectional dimension（0.36, 0.41, 0.46 mm） were selected. Box loops, double horizontal loops, and double vertical loops were manufactured according to established parameters, and loading experiments were carried out with a micro-orthodontic archwire testing apparatus. Results When the cross-sectional dimension and the span were identical, the appliance stiffness of the box loops, double horizontal loops, and double vertical loops increased successively. Conclusion Configuration of loops exert a great influence on appliance stiffness, and the mechanical properties of box loop are optimal. The force generated by box loop was lighter, slowly decreasing, and easier to be controlled. The activation range of box loop（0.41 mm） was appropriate when the produced optimal moment was between 0.01～0.02 N·m. Box loop（0.41 mm） is preferred to rectify the individual tooth with mild or moderate mesiodistal displacement.
