The cause of virulence decline of hypovirulent isolate Ep-1PN ofSclerotinia sclerotiorum (Lib.) de Bary has been studied through transmitting test and analysing its offsprings derived from single ascospore. When the colony of a normal strain Ep lPNA 183 intermingled with the colony of Ep-lPN, its margin became abnormal, just like EplPN. Cultures isolated from toptip of the colony of Ep-1PNA 183 showed the hypovirulence and its associated properties of Ep-1PN. Moreover, a series of cultures which have characteristics between EplPN and normal strain could be isolated from stipes of Ep-1PN. More than 200 offsprings derived from single ascospore of Ep-1PN were normal strains. According to those results, there exists a transmissible cytoplasmic hypovirulent element in hypovirulent isolate EplPN of S.sclerotiorum.