Abstract: OBJECTIVE To study the changes of chemical composition in Rehmannia glutinosa during the growth process. And to determine the correlation between the accumulation trends of main chemical composition and changes of organizational structure. To determine the best harvesting time of Rehmannia glutinosa. METHODS Content of catalpol and verbascoside in Rehmannia glutinosa were determined by HPLC; monosaccharide and oligosaccharide in Rehmannia glutinosa were determined by the method of RP-HPLC-RID; polysaccharide in Rehmannia glutinosa were determined by the method of phenol-sulfuric; amino acids in Rehmannia glutinosa were determined by automatic amino acid analyzer. SPSS software were used to have the principal components analysis for each chemical composition to observe the changes of organizational structure through the paraffin section and freehand section. SPSS software were used to analysis the correlation between the content of chemical composition and organizational structure. RESULTS There were significant correlations between the content of catalpol and the proportion of the xylem, the cortex; the content of polysaccharide and the proportion of the phloem;the content of stachyose and the proportion of the xylem, the phloem of xylem, the cortex, the numbers of secretory cell in 85-5. There were significant correlations between the content of amino acid and the proportion of the cortex, the numbers of secretory cell in Beijing-Ⅲ. CONCLUSION The content of chemical composition in root tuber of Rehmannia glutinosa is higher from late October to early November, during this period, the proportion of phloem in root tuber is smallest and the proportion of xylem in root tuber is biggest, it is coincided with the traditional harvest of Rehmannia glutinosa.