A new type of α-galactooligosaccharide( α-GOS) by microbial fermentation from herba lycopi was studied.Its componet includes melibiose,trisaccharide,tetrasccharide and pentosaccharide.By Plackett-Burman design and response surface methodology,the optimization of preparation α-GOS by fermentation was carried on.A Plackett-Burman design was first used to evaluate the influence of six related factors,and fermentation temperature, concentration of initial sugar,yeast extract and KH 2 PO 4 were selected as the important factors.Steepest ascent experiment and Box-Behnke design were used for further optimization on these factors.By establishing the regression model equation,the optimal concentration of these variables were determined as: fermentation temperature 28℃,concentration of initial sugar 8.76° BX,yeast extract 1.2g /100ml,KH 2 PO 4 0.21g /100ml.Under the above optimal culture conditions,the concentration of α-GOS increased from 86% to 94%.