  Kontsevich and Soibelman introduced a notion of orientation data on
Calabi-Yau category. It can be viewed as a consistent choice of spin structure
on moduli space of objects in the given category. The orientation data plays an
important role in Donaldson-Thomas theory. Let X be a projective, simply
connected and torsion free CY 3-fold. We prove the existence of orientation
data for the moduli space of coherent sheaves on X.
