The present invention is related to a novel process for preparing an optically active 5-hydroxy-3-oxo-hexanoic acid derivative or its tautomer which is useful intermediate for preparing statins such as atorvastatin and rosuvastatin. Blaise reaction of (S)-4-halo-3-hydroxybutyronitrile with -haloacetate is utilized as a key reaction to provide the product with minimal formation of side products and in good yield.