The aim of this study was to compare and quantify the mechanical properties of commercially available Nickel- Titanium open coil springs supplied by five different companies. Materials and methods: sixteen groups of springs, each comprised ten springs (totally 160 springs) from five manufacturers were tested 4 times over a 4 week period. A universal testing machine was used to measure the force characteristics when the springs were compressed to half of their original length and then when gradually allowed to decompress. Results: Both Dentaurum strong and Ortho organizer 0.35 x 0.76 springs showed the significantly highest mean maximum force values among all groups. On the other hand, 3M Unitek light spring showed the significantly lowest mean maximum force value among all groups. For all spring types, the maximum force value changed over time and was unpredictable. Conclusion: The clinician should be aware of the force values produced by the open coil springs to assure that these force levels is within the acceptable range in order to secure uncomplicated orthodontic tooth movement. Moreover, the manufacturer's methods for determining the labeled force values of springs require clarification.