  We find that the mixture of Ramond-Ramond fields and Neveu-Schwarz two form
are transformed as Majorana spinors under the T-duality group $O(d,d)$. The
Ramond-Ramond field transformation under the group $O(d,d)$ is realized in a
simple form by using the spinor representation. The Ramond-Ramond field
transformation rule obtained by Bergshoeff et al. is shown as a specific simple
example. We also give some explicit examples of the spinor representation.
