The motion of incompressible electrical conducting fluids can be modeled by magnetohydrodynamics equations, which consider the Navier-Stokes equations coupled with Maxwell's equations. For the classical Navier-Stokes system, there exists an extensively study of the convergence rate for the Galerkin approximations. In this work, we extend the estimates rates of spectral Galerkin approximations for Navier-Stokes equations to the magnetohydrodynamic equations. We prove optimal error estimates in the L 2 (Ω) and H 1 (Ω)-norms and obtain a result similar to that one of Rautmann for the H 2 (Ω)-norm for the Navier-Stokes case. In this sense, we reach basically the same level of knowledge as in the case of the classical Navier-Stokes.