Abstract: Shishanzi site (around 7000aB.P.,33°51'N,116°49'E),located in Suixi County,Anhui Province,China,is one of the representative mid-Neolithic sites situated in the middle range of Huai River. In order to investigate the ancient human diet and the level of economic development of ancient people living at this site,we performed starch grain analysis of the surface residues of five stone plates and five stone rods excavated from the site. The study of the organic residues showed a rich variety of plant starch grains on the surface of these stone tools. More than 1900 starch grains with cross-extinction features have been identified. These ancient starch grains could be divided into at least 8 categories and 10 sub-categories according to their morphological features. The possible sources of these starch grains could be at least eight plant species,including job's tears(Coix lacryma-jobi),the wheat tribe(Triticeae dumort),yam(Dioscorea opposita),lotus root(Nelumbo nucifera),lotus(Nelumbo nucifera),cowpea genus(Vigna),avena(Avena),the ginger family(Zingiberaceae). In addition,some of the starch grains might come from unknown species of Gramineae plants. The diversity of starch grains found within the surface residues of the stone tools from Shishanzi site reflects the variety of plant species processed by the stone tools and the multiplicity of ancient human's plant food resources. Statistical analysis of our data indicates the following. Firstly,regardless of the absolute number or the frequency of the occurrence,most of the starch grains found on the tools originate from job's tears,the wheat tribe and unknown species of gramineae plants. Therefore,these three plant species should be considered as the main objects processed by the stone tools. Secondly,the presence of starch grains originating from wheat tribe,yam,lotus root etc.on the stone tools indicates that most of the utilized plant species were collected rather than cultivated. It confirms that the gathering economic was still the main surviving mode of ancient humans living in Shishanzi site,and the development of agricultural economy lagged behind. Thirdly,this study shows that Shishanzi site was rich in natural resources. In particular,the presence of starch grains from aquatic plants such as lotus root,lotus and terrestrial plants such as gramineous plant,yam,indictates that there were rivers,lakes and other water resources in the site and its surrondings. The abundant water and plant resources together with the warm and humid megathermal climate created perfect conditions for the development of gathering economy in the region of Shishanzi site. Finally,the data shows that our ancestors in the middle Huai River may have been gathering Avena already around 7000aB.P. In general,our study elucidates,to a certain extent,the utilization of plants in the middle Huai River region in the mid-Neolithic age. The findings provide the scientific basis for the study of survival mode of ancient humans as well as the origin and early development of primitive agriculture in the middle Huai River region in the mid Neolithic age. In addition,this study shows that starch grain analysis can be a reliable method of research and can provide the scientific basis for agricultural archaeology and ancient environment reconstruction.