Dielectric and electrostrictive properties of 0.99Pb(Mg1/3Nb2/3)O3-0.01Pb(Mn2/3W1/3)O3 system using KCl-NaCl salts have been investigated as a function of the amount of excess MgO. For the specimens calcined at $700^{circ}C$ for 1h and then sintered at 115$0^{circ}C$ for 4h, the perovskite phase and the relative density were about 98% and 95%, respectively. With the addition of excess MgO up to 10mol%, the dielectric constant and the electrostrictive strain increased due to the increase of density and grain size, and these properties were degraded by the further addition of excess MgO. For the 10mol% excess MgO added specimen, the maximum electrostrictive strain, Xmax, and the electrostrictive coefficient, Q11, were 4.5$ imes$10-4, 3.5$ imes$10-2m4/C2, respectively. These results were correlated with the property of PMW and microstructure.