Light-response parameters of plant photosynthesis are important indices characterizing ecosystem productivity. Gross primary production (GPP) is one of the most basic energetic and biogeochemical characteristics of an ecosystem. The study of plant photosynthesis at the canopy and ecosystem scales is improved by using micrometeorological methods such as the eddy covariance technique. To investigate photosynthesis and its light-response characteristics, CO₂ flux was continuously measured using the eddy covariance technique in a summer maize field on the North China Plain from 2003 to 2006. Net ecosystem carbon exchange (NEE) rose with increase in photosynthetically active radiation and their relationship was expressed by a rectangular hyperbola function. Light-response parameters and GPP showed temporal variation and generally peaked at the heading/grain filling stage. The seasonal mean initial light use efficiency (α), maximum photosynthesis rate (P_max) and dark ecosystem respiration (R_d) in the summer maize field varied from 0.054 to 0.124 μmol/μmol, from 1.72 to 2.93 mg CO₂ · m^-2 · s^-1, from 0.23 to 0.38 mg CO₂ · m^-2 · s^-1, respectively. Light-response parameters were significantly affected by maize growth. Monthly mean α, P_max and R_d rose exponentially with the increase in leaf area index (LAI). About 50% of the changes in α,P_max and R_d can be ascribed to the variation in LAI. Usually, weak light reaching the ground is mainly composed of scattered light. Under a large LAI, a suitable canopy structure was advantageous for absorbance of scattered light from all directions and led to a high use efficiency of weak light. In summer, the temperature was high and varied within a narrow range. The influence of temperature on R_d was insignificant. LAI was the main factor controlling R_d. Total seasonal GPP in the maize field was 806.2, 741.5, 703.0 and 817.4 g C/m² in 2003, 2004, 2005 and 2006, respectively. The interannual differences in GPP were largely attributable to climatic variation. Compared with the other years, the low GPP in 2005 was a result of the lower soil water content. Monthly mean GPP increased exponentially with an increase in air temperature (P < 0.01). Air temperature explained 34%-51% of GPP variation. During the vegetative growth period, GPP increased significantly with increasing LAI (P < 0.01) and the relationship between GPP and LAI was expressed by a rectangular hyperbola function. During the reproductive growth period, GPP decreased exponentially with declining LAI (P < 0.01). Under the same LAI, GPP during the reproductive growth period was lower than that during the vegetative growth period, because of the decline in leaf chlorophyll content and the lower air temperature during the reproductive growth period.