[en] We have studied some phenomenological aspects of the B meson physics by using lattice QCD, which is a non perturbative method (based on the first principles of Quantum Field Theory) of computing Green functions of the theory. Pionic couplings g₁ and g₂, parameterizing the effective chiral Lagrangian which describes interactions between heavy-light mesons and soft pions, have been computed beyond the quenched approximation (at N_f = 2). We have renormalized the operator q-barγ_μγ⁵q non perturbatively by using chiral Ward identities. We obtain g₁ = 0.4/0.6 and g₂ = -0.1/-0.3. We have estimated from an un-quenched simulation (at N_f = 2) the strange quark mass: the non perturbative renormalisation scheme RI-MOM has been applied. After the matching in the MS scheme the result is m_s(2 GeV) = 101 ± 8(-0,+25) MeV. We have proposed a method to calculate on the lattice the Heavy Quark Effective Theory form factors of the semileptonic transitions B → D^** at zero recoil. The renormalisation constant of the operator h-barγ_iγ⁵D_jh has been computed at one-loop order of the perturbation theory. We obtain τ_1/2(1) = 0.3/0.5 and τ_3/2(1) 0.5/0.7. Eventually the bag parameter B_Bs associated the B_s - B_s-bar mixing amplitude in the Standard Model has been estimated in the quenched approximation by using for the strange quark an action which verifies the chiral symmetry at finite lattice spacing a. Thus systematic errors are significantly reduced in the renormalisation procedure because the spurious mixing of the four-fermion operator h-barγ_μLqh-barγ_μLq with four-fermion operators of different chirality is absent. The result is B_Bs = 0.92(3). (author)