Abstract: The purpose of this research was to produce L- Lactic acid by Lactobacillus rhamnosus in semi- solid state fermentation and develop low- cost feed acidifiers. Firstly,single factor test,Plackett- Burman design and response surface methodology were employed combined to optimize the cultural medium of L- Lactic acid production.Secondly,the fermentation kinetics of this semi- solid state fermentation was analyzed. The results showed that the optimal cultural medium was composed of corn flour 18%,soybean meal 10.35%,bran 4.5%,yeast0.53%,sodium acetate anhydrous 0.5%,K2HPO4·3H2O 0.3%,ammonium citrate 0.3%,Mg SO40.058%,Mn SO40.025% and glucose 1.2%,initial water content 70%( w / w). The yield of L- Lactic acid was increased to( 120.82 ±1.60) g / kg( wet weight) that improved by 56.16%.The maximum specific cell growth rate was( 0.0951 ± 0.001) h- 1,specific L- Lactic acid production rate was( 1.02 ± 0.01) h- 1,and special reducing sugar consumption rate was( 0.0855 ± 0.009) h- 1during the semi- solid state fermentation of optimized medium.The high production of L- Lactic acid by Lactobacillus rhamnosus in semi- solid state fermentation can be developed as a new feed additive combined with feed acidifier and inactivated Lactobacillus cells.