By applying conditional variance modeling and Granger causality test, we specified some important determinants of real and nominal uncertainty in Iran (1960-2006). In addition we investigated the negative effects of these uncertainties on output growth. Our results show that an increase in the rate of inflation and a decrease in the level of oil revenues can raise nominal uncertainty, while real uncertainty is arisen by oil revenues. This study could not reject the hypothesis that nominal and real uncertainty has no effect on output growth.