Abstract: Abstract: Huang-huai Plain of China is featured of heavy rainfall in June-September, which accounts for 60%-70% of total annual precipitation. The heavy rainfall-induced soil waterlogging stress during the summer rainy season (June-September) is one of the major constraints for summer maize (Zea mays L.) production in the region. It is necessary to determine the critical days that crops can tolerate stress for minimizing the adverse effect of stress on crop production. In this study, a field experiment was conducted during the summer maize growing season from 2011 to 2012 at Hefei Agrometeorological Experimental Station in Anhui province in order to investigate the effect of waterlogging stress on the growth and grain yield of summer maize and determine the critical days of waterlogging stress. The Xundan No.20 widely cultivated in Huang-huai Plain was used as tested maize. Surface (3 and 5 days) and subsurface (5, 7 and 10 days) waterlogging duration the jointing and tasseling stages were designed. The surface waterlogging was assumed to be 5 cm depth of water on soil surface and the subsurface waterlogging was considered to be 90% of field capacity. Meanwhile, the control soil (CK) had soil moisture kept at 70%-80% of field capacity. Each waterlogging treatment in a plot about 20 m2 had three replicates The aboveground dry matter and leaf area per plant were measured at the jointing, tasseling, milking and maturity stages, respectively. Yield characteristics of summer maize, such as ear length, ear diameter, length of bare tip, grain number per spike, 100-seed weight and harvest index were determined. The results showed that the effect of waterlogging on the growth and yield varied with types and duration of waterlogging at different developmental stages of summer maize. Compared with CK, the surface waterlogging for 3 days at the jointing and for 5 days at tasseling stages resulted in an average decrease in yield with 28.4% and 42.8%, while the subsurface waterlogging for 5 days at jointing or tasseling stages resulted in an average decrease in yield with 3.0%(2012)-13.8%(2011) and 3.4%(2012)-5.5%(2011), respectively. The damage degree due to waterlogging stress at the jointing stage was greater (P<0.05) than at the tasseling stage under the same waterlogging duration, and the yield loss rates were increased with the prolonged waterlogging days. The yield loss rates of surface waterlogging for 5 days was 3.1 times that of surface waterlogging for 3 days at jointing stages, while the yield loss rates of subsurface waterlogging for 10 days were 1.3-3.0 times (2011) and 2.4-3.2 times (2012) that of subsurface waterlogging for 5 days, respectively. The yield losses were related with 100-grain weight and grains per spike. Waterlogging stress not only reduced the grain yield of maize, but also affected the characters of spike, harvest index, the accumulation and distribution of aboveground dry matter weight, leaf area per plant and the grain filling rates. Leaf areas per plant at the milky stage were decreased by 21.2%-65.7% and 4.0%-8.5% in the surface waterlogging and subsurface waterlogging treatments, respectively. The aboveground dry matter weight were reduced by 43.6% and 6.5% in the treatments of surface waterlogging for 5 days and subsurface waterlogging for 10 days at jointing stages, respectively, and the proportion of dry matter in corncob was reduced by 18.8% and 4.2%. The grain filling rate at 25-35 days after silking in the surface waterlogging for 5 days at the jointing or tasseling stages and subsurface waterlogging for 10 days at the jointing stage was decreased by 59.6%, 50.0% and 28.9%, respectively. Therefore, effective measures should be taken to avoid waterlogging for 3 days at jointing stages or 5 days at tasseling stages, when heavy rainfall occurred in the growing season of summer maize.