Abstract: Field experiments were conducted in 1992 and 1993 in Beijing and Tangshan regions, two representatives of the Haung-Huai-Hai region, to study the functional relationships between the relative weedy or weedfree days of naturally infecting weeds and the relative yield of summer maize. Models for calculating the eco-economic threshold period for controlling the weeds in maize feild were established. The results showed that the eco-economic threshold period for controlling the naturally infecting weeds in summer maize feild ranged from 11.2% to 50.9% of the total growth days of maize after crop emergence in this region. This is the critical period to remove all of the weed damage and to maximally utilize the role of the weed flora in reducing the loss of water, nutrients and soil during the growth season of maize in this region.