The correction factor P(t) used in determining the fast fission effect of 238 U was measured in the range of cooling time t=30-200 min for irradiations of 30 min, 2 and 5 hr, respectively. This factor depends quite sensitively on the bias level in r-ray counting as well as on the irradiation time and cooling time after irradiation. It is emphasized that neglect of the dependence of P(t) on irradiation time and bias level results in a large error in measurements of fast fission effect of 2asu.