The Σ 0 − Λ mixing angle in isospin-asymmetric nuclear medium is investigated by using QCD sum rules. From the general consideration of the inmedium baryonic correlations, in-medium baryon mixings are shown to have several Lorentz structures such as the scalar mixing angle θ S and the vector mixing angle θ V . This causes a difference between the particle mixing θ (= θ S +θ V ) and the anti-particle mixing θ (= θ S − θ V ). From the finite energy sum rules for the Σ 0 − Λ mixing, we find that the in-medium part of the mixing angle has a relation θ S Med ∼ −θ V Med in the isospin-asymmetric medium. This implies that the medium affects mainly the anti-particle mixing. From the Borel sum rules, we obtain | θ − θ 0 | ≃ 0.39 |(ρ n − ρ p )|/ρ 0 with θ 0 , ρ n , ρ p and ρ 0 being the vacuum mixing angle, the neutron density, the proton density and the normal nuclear matter density respectively.