The diffusion coefficient and conductivity of the chemically cross-linked polymer gel electrolyte composed of poly(ethylene glycol.) dimethacrylate and LiBF4-EC/EMC were measured in order to investigate the effect of the polymer on the nature of carrier migration in the gel. With an increase of the polymer fraction in the gel, the dissociation degree of the salt in the gel increased. This indicates that the polymer accelerated the dissociation of the salt in the progress of gelation. The dissociated cation and anion showed a different manner of change in the activation energy of diffusion with gelation. The cation, through Coulombic interaction, showed a decrease in activation energy, revealing a change in the migration mechanism to hopping on the oxygen sites linked with the segmental motion of the chains. The anion, on the other hand, showed an increase in the activation energy of diffusion. This means that the conduction mechanism essentially follows, as in solution, the mobility of the solvent correlated with enhanced viscosity due to gelation.