Abstract: By applying MBTR-1 dynamometer the deformative force of various labial arehcs and springs made of stainless steel wire and nickel- titanium wire was determined.It proved that the deformative force of nickel-titanium wire was about 1-5 times lower than that of the stainless steel wire.It also suggested that 3 reqirements should be futfilled for the orthodontists to select the suitable kind of wire during designing an orthodontic appliance,