The director distributions in thin nematic slabs with parallel structure have been calculated as a function of the applied electric field. They are different from the cells with the anti-parallel structure, in that three solutions exist in a specified voltage range which predict a transition of the director deformation from splay to bend configurations in the parallel structure. This splay-bend transition has been confirmed experimentally by a capacitance-voltage (C-V) hysteresis method. The C-V characteristics showed a hysteresis around the deformation transition and this magnitude corresponded to the facility of a splay-bend transition: a comparison of these hysteresis characteristics for different materials enables us to evaluate the splay-bend transition speed easily.