Free surface effects on rotational deformation mediated by grain boundary dislocations in nanocrystalline materials are theoretically described. The critical stresses and characteristic geometric parameters for the rotational deformation occurring in nanocrystalline materials near their free surfaces are calculated and compared with those specifying the rotational deformation in bulk regions. The role of the free surface effects in the interpretation of electron microscopy data for plastically deformed nanocrystalline materials is discussed.