The gassing tendency of aromatic dielectric liquids used for high voltage power capacitors was measured as varying several parameters such as temperature, electrical stress, moisture content, and aromaticity. The effect of moisture on the gassing tendency of some kinds of hydrocarbons which have different aromaticity was measured in advance to determine the upper limit of moisture content in these capacitor oils. The partial discharge of capacitors impregnated with previously mentioned dielectric liquids was also examined to ascertain the relationship between the gassing tendency and partial discharge of capacitors. The typical aromatic oils of alkyl benzene, alkyl naphthalene containing a certain amount of alkyl anthracene, and benzyl toluene containing dibenzyl toluene were impregnated into capacitors after being purified according to the procedure for electrical insulation oils. Both of the aromatic type and the amount of aromatic carbon seemed to play an important role to reduce partial discharge capacitors.