The existing experimental results indicated that many extrinsic and intrinsic factors have a remarkable influence on the preference of deformation twinning behavior, such as, SFE, grain size, deformation mode, strain, strain rate, temperature and so on. However, there still exist some obvious contradictory conclusions. In order to well understand those issues, some special experiments were designed to study the deformation twinning behavior under the identical external deformation conditions. The investigation shows that crystallographic orientation and SFE have a remarkable influence on the process of deformation twinning in FCC crystals. For Al single crystal, only microtwins or stacking faults were found to nucleate although a preferential crystallographic orientation was selected, indicating that SFE plays a critical role in deformation twinning. While for Cu single crystal, abundant deformation twins were observed although it was extruded for only one-pass at RT and low strain rate, demonstrating that crystallographic orientation is one of the important controlling factors too. Whereas for Cu-3%Si alloy, deformation twins were only found in some grains, and others were full of planar dislocations although it has the lowest value of SFE, showing further that deformation twinning is controlled by the combined effects of SFE and crystallographic orientation. The further analysis based on the fundamental dislocation mechanisms indicates that twinning stress is mainly controlled by three intrinsic factors, such as, SFE, crystallographic orientation and grain size. With the decrease in grain size, the twinning stress continually increases and the influence of SFE is gradually reduced; whereas the crystallographic orientation always plays an important role in deformation twinning. (See Han WZ et al., Phys. Rev. Lett. (2008) 115505, Appl. Phys. Lett. 92 (2008) 221909, Philos. Mag.88 (2008) 3011 and Adv. Eng. Mater. 2008)