In order to study the deformation mechanisms during ultrahigh strain rate deformation of face centered cubic metals, laser shock peening of a 304L stainless steel is systematically investigated. Two deformation modes—microtwins and microbands—and their interrelationship during high strain rate deformation are discussed in detail. Transmission electron microscopy and selected area electron diffraction are employed to study the deformation modes. It is found that twinning takes place even when the shock pressure is much less than the critical twinning stress in stainless steels. Theoretical critical twinning stress is not the only criteria to decide the deformation modes of twinning or slip. The formation of twinning and slip can be affected by the factors such as loading profile, loading stress/strain rate, stacking fault energy, grain sizes, and cell substructures. Factors that influence twin-slip transition in shock loading are discussed. The formation of dislocation structure is compared with those predicted using 3D dislocation dynamic simulation.