  Mixing of 56Ni, whose nuclear decay energy is a major luminosity source in
stripped-envelope supernovae, is known to affect the observational properties
of stripped-envelope supernovae such as light-curve and color evolution. Here
we systematically investigate the effect of 56Ni mixing on the photospheric
velocity evolution in stripped-envelope supernovae. We show that 56Ni mixing
significantly affects the early photospheric velocity evolution. The
photospheric velocity, which is often used to constrain the ejecta mass and
explosion energy, significantly varies by just changing the degree of 56Ni
mixing. In addition, the models with a small degree of 56Ni mixing show a
flattening in the early photospheric velocity evolution, while the fully mixed
models show a monotonic decrease. The velocity flattening appears in both
helium and carbon+oxygen progenitor explosions with a variety of ejecta mass,
explosion energy, and 56Ni mass. Some stripped-envelope supernovae with early
photospheric velocity information do show such a flattening. We find that Type
Ib SN 2007Y, which has early photospheric velocity information, has a signature
of a moderate degree of 56Ni mixing in the photospheric velocity evolution and
about a half of the ejecta is mixed in it. The immediate spectroscopic
follow-up observations of stripped-envelope supernovae shortly after the
explosion providing the early photospheric evolution give an important clue to
constrain 56Ni mixing in the ejecta.
