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tokens: ; Taddia et al. 2018 ; Prentice et al. 2019 ). Indeed, the previous studies on 56 Ni mixing in stripped-envelope SNe indicate that 56 Ni mixing can affect the photospheric velocity evolution in stripped-envelope SNe (e.g. Dessart et al. 2012 ). In this paper, we perform a systematic study of the effect of 56 Ni mixing on the photospheric velocity evolution in stripped-envelope SNe. The rest of this paper is organized as follows. We summarize how we get progenitor models, how we mix 56 Ni, and how we conduct radiation transfer simulations in Section 2 . Our results are presented in Section 3 . We discuss the results and conclude this paper in Section 4 . 2 METHODS 2.1 Progenitor models We obtain hydrogen-free progenitor models by using the public stellar evolution code mesa (Paxton et al. 2011 , 2013 , 2015 , 2018 , 2019 ). We assume a metallicity of Z = 0.02. We adopt the standard mixing-length theory for convection with a mixing-length parameter of 2.0 with the Schwarzschild criteria. Our focus in this paper is on hydrogen-free SNe. There are many mechanisms to remove the hydrogen-rich envelope from SN progenitors and the mass-loss mechanisms themselves are a well investigated field of research (Smith 2014 ). In this work, we artificially remove hydrogen-rich envelopes to obtain hydrogen-free SN progenitors. We first evolve massive stars from zero-age main sequence (ZAMS) to the end of helium burning without mass loss. The helium core mass does not change much after the helium burning. At this point, we stop the stellar evolution calculation and gradually reduce the mass from the surface through wind by using the relax_mass option in mesa . The maximum mass-loss rate allowed in this process is 10 −4 M ⊙ yr −1 ( lg_max_abs_mdot = -4 in mesa ). When the stellar mass reaches the desired mass, we resume the stellar evolution calculation without mass loss. The stellar evolution calculations are performed at least until the end of oxygen burning. The subsequent evolution would not affect the envelope structure of the progenitors significantly. Using the method described above, we obtain three hydrogen-free progenitor models: He3.85, CO3.85, and He4.90 (Table 1 ). The density structure of the progenitors is shown in Fig. 1 . He3.85 and CO3.85 have the same mass (3.85 M ⊙ ), but He3.85 is a helium star while CO3.85 is a carbon+oxygen star without helium. The ZAMS masses of He3.85 and CO3.85 are 14 M ⊙ and 20 M ⊙ , respectively. He4.90 is a 4.90 M ⊙ helium star progenitor with a ZAMS mass of 17 M ⊙ . These masses are determined to have the typical ejecta mass (≃2 M ⊙ ) in stripped-envelope SNe (e.g. Lyman et al. 2016 ). Figure 1. Density structure of the progenitor models in the mass coordinate (top) and radius coordinate (bottom). Table 1. Progenitor properties. When we explode these progenitors, we set a mass cut M cut below which is assumed to form a compact remnant that is not ejected. The mass cut is set at 1.35 M ⊙ (He3.85 and CO3.85) and 1.40