  The bound on the time variation of the fine structure constant ($\alpha$)
driven by the dynamics of quintessence scalar field which is coupled to
electromagnetism is discussed using phenomenological quintessential models
constrained by SNIa and CMB observations. We find that those models allowing
early quintessence give the largest variation $\Delta\alpha$ at the decoupling
epoch. Furthermore, the fifth force experiments imply that
$\Delta\alpha/\alpha$ is less than about 0.1%.
