Abstract: The critical period for weed control (CPWC) is a key consideration in integrated weed management programs. This study aimed to determinate CPWC in no-tillage summer maize fields and to discuss the internal mechanisms of weed control. The results were pivotal in designing management strategies to minimize weed infestation during critical periods of crop development. Field studies on the dynamics of weed community and CPWC were conducted in 2010 and 2011 near Shijiazhuang in Hebei Province. Weed densities were investigated in 1 m × 1 m quadrats within field plots. Two treatments of weed infestation and weed-free were adopted in no-tillage summer maize fields. The results showed that the main species of the weed community belonged to the grass family, with a relative density above 94.93%. Eleusine indica (L.) Gaertn. and Digitaria sanguinalis (L.) Scop. were the dominant species of the weed community. Weed density increased until 21 d after maize planting and continuously decreased thereafter. Weed germinating during 0~20.41% of the maize growth period heavily influenced maize yield, with an increasing yield loss of 28.54%~37.51%. Weed infestation was most intensive during the 19.09%~42.73% of the growth period, with an increasing yield loss of 27.00%~46.22%. The best-fit curve between relative time of weed infestation (start time of CPWC) and relative yield of maize was a modified Logistic model. Also the best-fit curve between relative time of weed-free (end time of CPWC) and relative yield of maize was a Gompterz model. The models had high coefficients of determination (R² > 0.99). Based on the models, CPWC in no-tillage summer maize field was 14.15%~56.62% of the growth period; with an acceptable relative yield loss of 3%. However, CPWC was likely to be slightly affected by weed density.