Field experiments were conducted to explore the relationship between canopy apparent photosynthesis (F’N,can) ^nd grain yield of maize {Zea mays L.) and to determine over two years the effects of plant population density (PPD) on /'N.can- Yedan No. 4 hybrid (vertical leaf type) were grown at fíve PPD in 1989 and 1990. Measurements of /"n can were made throughout the growth period in a large plastic covered chamber put over a segment of the canopy. Maximum values of /^N.can ca. 1500 pmol(photon) m'^ s'* were about 2.8 mg(C02) m‘2(soil area) s'* at the anthesis stage and they decreased with age throughout grain filling. Differences in /^N can between PPD could be attributed to leaf area index (LAI) or radiant energy interception. There was a quadratic regression between and LAI. The critical LAI for Pn can was about 4 to 6 for Shendan No. 7 and Yendan 4, respectively. Grain yield also differed between PPD, the range was 850 to 1140 g m'^ in the two cultivars. Increasing PPD of Yedan No. 4 up to about 9 plants per m^ and decreasing PPD of Shendan No. 7 to about 4 plants per m^ in a non-stressed environment should increase grain yield. Pfj can anthesis was positively correlated with biological {r = 0.8750** to 0.9797**) and grain (r = 0.8932** to 0.9914**) yields.