In this paper, using monetary policy rules, we build a model which describes the fixing of the interest rate by the Bank of Central African's States (BEAC). First, with a GMM adapted for a forward looking rule, we propose a reaction function for this central bank. The result shows that from 1986 to 2006, the formulation of the monetary policy strongly depends on the past interest rate. The rule describes well the interest rate fixation process by the BEAC which tends to be guide mainly by price stability than growth objectives. Secondly, we estimate a simple rule using an error correction model. In this case, the results were better but confirm those issued by the GMM method.