(Research Laboratory, Tomoegawa Paper Mfg. Co. Ltd., Mochimune, Shizuoka, Japan) Effects of di-valent cations on dielectric properties of oil-impregnated paper were studied in the same method as that of part 2.(1) Effects of di-valent cations on the dielectric loss at low temperature (about 20°C) were hardly remarkable, but Ba++ and Ca++ influenced badly a little.(2) Di- valent cations gave remarkable influence on the dielectric loss at high temperature of oil-impregnated paper, but those did not influence more badly than mono-valent cations.(3) In all the di-valent cations used, Ph++ was the best.(4) It was difficult that the dielectric properties of oil-impregnated paper were improved without washing with water after immersed in the aqueous solution of di-valent cations, and only the papers treated with the dilute aqueous solution of Pb-salts were improved.(5) The dielectric properties of the papers treated with sulphate were very excellent in the un-impregnated paper, but those were not very excellent in the oil-impregnated paper.