Abstract: In order to improve the industrial usability of flue-cured tobacco leaves,the variation tendencies of main chemical components(contents of nicotine,total N,reduced sugar,K,Cl,starch) in middle leaf samples(C3F) from main tobacco growing areas in China during 2002-2013 were analyzed.The results showed that:1) The content ranges of nicotine,total N,reduced sugar,K,Cl,and starch in middle leaves were 1.38%-3.77%,1.35%-2.96%,16.96%-32.76%,1.09%-3.07%,0.11%-0.79%,and 2.00%-8.07%,respectively.2) The contents of starch,K and Cl in middle leaves were relatively stable over the years,however,reduced sugar content increased from 21.79% in 2002 to 26.54% in 2013,and increased obviously from 2005 to 2008.3) The average nicotine content decreased from 2.72% in 2002 to 2.29% in 2013 and obviously decreased from 2003 to 2007,and total N content changed in a way similar to that of nicotine.4) The changes of both tobacco planting areas and cultivars did not closely correlated to the changes of chemical components.The raising of reduced sugar content was mainly attributed to its increase in the areas other than north tobacco growing areas,while the declining of the contents of nicotine and total N were mainly relevant to their decrease in the upper and middle reaches of the Yangtze River,southwest and southeast tobacco growing areas.