The tribe-charging characteristics of powder in a rotating vessel are investigated under different humidities to clarify the effect of charge relaxation of powder. The sample powder was prepared through coating of ferrite carrier particles with silicone resin. t is found that the absolute value of the maximum charge of powder decreases sharply with increasing relative humidity below 30%, while the change is relatively small above 30%. The contact potential difference against a reference electrode is dependent on the relative humidity both for the powder and the wall material of the vessel, while the contact potential difference between the powder and the wall material is found to be constant, even if the humidity is varied. The maximum charge of powder is predicted well by using the conventional time constant that is obtained as the product of dielectric constant by the resistivity of the powder. However, the tendency of the charging characteristic curve can not be predicted as far as the conventional time constant is used. On the other hand, the charging characteristic curve can be predicted well using the time constant obtained by the null current potential curve in the measurement of contact potential difference.