[en] In this work quarkonia states are described as eigenstates of a Hamiltonian that allows the coupling of QQ to a pair of heavy-light mesons (Qq, Qq). The decay amplitude of QQ into Qq, Qq is derived using the flux tube breaking model. The coupled channel mixing effect is calculated both relativistically and nonrelativistically. The results shows that the relativistic effect in the coupled channel mixing calculation is very important. The relativistic effect makes the mass shifts of all states much larger than the nonrelativistic counterpart and relativistic treatment gives a results which is in better agreement with the experiment data. Relativistic and coupled channel mixing effects are taken into account to estimate the one photon radiative M1 and E1 transitions. The GRR potential model is used for this calculation. The decay rates of M1 transitions involving D-states are very small except the transition 1¹D₂ → 1³S₁ + γ which is about 2.13 kev. The S → S M1 transition rates are in agreement with or close to the experiment data after drop the term due to the scalar interaction. The result in chapter 5 shows that both the relativistic and coupled channel mixing corrections have large effect on the E1 transitions. For some E1 transitions with too large relativistic corrections the higher order correction are needed. The leptonic decay widths of D-states are discussed in section 5.6