The transient grating method using a nanosecond laser pulse as the pump and a CW laser beam as the probe is applied to a dye (methyl red)-doped isotropic liquid crystal(MBBA) under a de electric field. The diffracted signal decays biexponentially due to the faster (in the mu s time scale) heat diffusion and the slower (in the ms time scale) mass diffusion of the dye molecule at zero electric field. No significant changes of the heat diffusion by the electric field are observed. The decay by the mass diffusion is greatly affected by the electric field, but the electrohydrodynamic convective flow is not observed. The complementary grating effect is observed in the mass diffusion under the weak electric field : which is explained well with the hydrodynamic model. The viscosity of the medium and the solute-solvent interactions are affected by the electric field. The activation energy of diffusion, which corresponds to the activation energy of viscosity in the hydrodynamic model, decreases with the electric field. The intermolecular association of methyl red and MBBA increases under the electric field so that the diffusional friction grows. The viscosity decrease by th-electric field gives greater effect on the diffusion than the diffusional friction due to intermolecular association. Therefore, the diffusion becomes faster under the electric field.