The present invention relates to a process for preparing simvastatin, wherein the simvastatin dimer content is controlled. More particularly, the present invention relates to a process for preparing simvastatin having a simvastatin dimer content of about 0.2 to about 0.4% wt. The present invention also relates to a process for preparing simvastatin having a simvastatin dimer content of less than about 0.2% wt. The present invention also discloses a commercial scale process of preparing simvastatin having a specified simvastatin dimer content which is reproducible.