Optical resolution of (R,S)-2-hydroxy-4-Ethyl benzene [(R,S)-HPBE] by calcium alginate-glutaraldehyde cross-linking Serratia sp. lipase was developed, and the activity and many factors affecting the resolution were investigated. The optimum reaction conditions were temperature 50℃, pH=7.0, concentration of (R,S)-HPBE 40 mmol/L and ρ(lipase amount)=200 g/L. After 10 h of resolution by immobilized lipase, the conversion rate of (R)-HPBE is 93.4% and e.e. is 96.2%. After twelve times repeated batch operation, the conversion rate of product by immobilized lipase is more than 80%.