Optim™ fine is a new generation of ultrafine wool fibres, which are transformed by stretching and setting from merino wool. The Optim™ fine retains some of the physical and chemical properties of merino wool, but the morphology and other properties changes, such as the diameter, length, lusture, breaking extension and shrinkage. These changes results in the performance difference btween the fabric woven from Optim™ fine and fine wool fiber. To clearly understande the reason of the difference, the morphological and physical properties changes of Optim™ fine was examined in this paper, and the performance difference between wool fabric and Optim™ fine fabric was evaluated by Fabric Assurance by Simple Testing(FAST). It was found that the decreasing in breaking extension and increasing in shrinkage of Optim™ fine resulted in the declining in formability and dimensional stability of Optim™ fine fabric. Suggestions on the ways of constructing Optim™ fine into fabric were also given according to the anlysis of relationship between the properties changes of Optim™ fine fiber and that of its fabric.