Abstract: This invention relates to an improved process for preparing substantially pure simvastatin (I), chemically known as (1S,3R,7S,8S,8aR)-8-[2-[(2R,4R)-4-hydroxy-6-oxotetrahydro-2-H-pyran-2-yl]ethyl]-3 ,7-dimeth-yl-1,2,3,7,8,8a-Hexahydronaphthalen-1-yl2,2-dimethyl butanoate, which comprises of: a) treating lovastatin (II) with an alkali metal hydroxide in a chosen suitable alcoholic solvent followed by relactonization to obtain the diol lactone intermediate (III) in a single vessel. b) selective silylation of 4-hydroxy group of diol lactone intermediate (III) with a chosen suitable silylating reagent to obtain mono silylated intermediate diol lactone (IV).