In this study I make a Dynamic Stochastic General Equilibrium Model to study the monetary policy in Iran. Since monetary policy in Iran is not conducted using a specific rule, I first should modify the Taylor rule. The modified rule is such that the growth rate of monetary base is determined according to deviations of inflation and output from their target values. Although there are some inflation targets in Five-year Development Plans but there is no commitment to this targets. Therefore, we assume that there is implicit inflation targeting that is, there is inflation targeting for monetary authorities but the target value of inflation is not announced to public. Taking into account this assumption, monetary policy reaction function is estimated in a DSGE model. The empirical results show that there has been a kind of rule at the end of 70s and the beginning of 80s and that the implicit target has been higher than those of Development Plans. Dropping the modified rule from the model and solving it assuming discretionary and optimal monetary policies, the results suggest that in most periods the monetary policy has been conducted in a discretionary approach.