Abstract: Carbon dioxide evolution rate( CER) and respiratory quotient( RQ) are two important physiological parameters for deep understanding of bioprocesses. In this study,on the basis of L- lactic acid production by Lactobacillus paracasei,the relationships of CER and RQ with cell growth and metabolism were investigated. The results indicated that CER was closely related to cell growth rate,and it could characterize the cell growth status effectively.Furthermore,during the fermentation,intracellular metabolic pathway change could be well reflected by macroscopical RQ level.Through adoption of CER and RQ as online indicative parameters,and applying rational step wise nitrogen source addition strategy,overall L- lactic acid productivity and yield improved by 5.9% and3.0%,respectively,while the primary byproducts acetic acid and acetoin decreased by 31.3% and 24.7%. On the basis of these results,both of L- lactic acid quantity and quality could be efficiently improved through on- line monitoring physiological parameters CER and RQ.