This study consisted of two experiments to study the leaf photosynthetic pigment concentrations of cucumber. The first one studied the effects of elevated CO₂ and nitrate concentration under three nitrate concentrations [2(low N), 7(moderate N) and14(high N) mmol/L]and three CO₂ concentrations [400 (C1), 625 (C2) and 1 200 (C4) μmol/mol]. The second one studied the effects of elevated CO₂ and N forms under three CO₂ concentrations [400 (C1), 800(C3) and 1200 (C4) μmol/mol] and four ratios of nitrate to ammonium concentrations [14/0(N1), 13/1(N2), 11/3 (N3)and 8/6(N4)]. The results showed that: at the seedling stage, C4 treatment enhanced the biomass of all the three N supplies and this effect decreased at the initial fruit stage. The biomass of C3 treatment increased and was the highest among the CO₂ treatments. At the seedling stage, the chlorophyll a, b and carotenoids concentrations of the low and moderate N increased under C3 treatment, while high N increased the chlorophyll b and total pigment concentrations. Three pigment concentrations of N2 treatment were the highest under C3 treatment,while their concentrations in N3 treatment were the highest under C4 treatment among the CO₂ treatments. Thus, at the seedling stage, elevated CO₂ decreased three pigment concentrations of low N due to “dilution effect” caused by the high growth rate. But when N concentration was 14 mmol/L, elevated CO₂ increased the pigment synthesis and this rate was higher than the growth rate, which resulted in higher pigment concentrations. This effect also existed with high ammonium supply. The pigment concentration was generally controlled by the growth rate and pigments synthesis rate simultaneously. Under moderate and high N,chlorophyll a/b at the seedling stage increased under high CO₂, but only that of the high N decreased at the initial fruit stage. Moreover, C4 treatment enhanced chlorophyll a/b, which may be enhanced by high light density and low N concentration. Practically, the cucumber cultivation under elevated CO₂ should combine with high N concentration, high ammonium supply rates and high plant density.