In order to investigate the factors for attaining a high enantio-differentiating ability (e.d.a.) for the enantio-differentiating hydrogenation of methyl acetoacetate over a tartaric acid pre-modified reduced nickel catalyst, the relation between the coverage of the modifiers on a nickel surface and e.d.a. was investigated. A Ni773 or Ni1373 (the reduced nickel catalyst prepared from a nickel oxide which was calcined at 773 K or 1,373 K, respectively) was used for the preparation of a modified catalyst. It was revealed that the tartaric acid coverage of about 0.2, the appropriate surface for the enantio-differentiation, such as the conditioned surface of Ni1373, and the inhibition of the hydrogenation on non-enantio differentiating sites by Br-, were all important for attaining a high e.d.a. This would be interpreted that only when tartaric acid is adsorbed as an isolated molecule on the appropriate surface, the acid exerts its intrinsic e.d.a.