The responses of CO 2 assimilation to [CO 2 ] (A/C i ) were investigated at two developmental stages (R5 and R6) and in several soybean cultivars grown under two levels of CO 2 , the ambient level of 370μbar versus the elevated level of 550μbar. The A/C i data were analyzed and compared by either the combined iterations or the separated iterations of the Rubisco-limited photosynthesis (A c ) and/or the RuBP-limited photosynthesis (A j ) using various curve-fitting methods: the linear 2-segment model; the non-rectangular hyperbola model; the rectangular hyperbola model; the constant rate of electron transport (J) method and the variable J method. Inconsistency was found among the various methods for the estimation of the maximum rate of carboxylation (V cmax ), the mitochondrial respiration rate in the light (Rd) and mesophyll conductance (g m ). The analysis showed that the inconsistency was due to inconsistent estimates of g m values that decreased with an instantaneous increase in [CO 2 ], and varied with the transition C i cut-off between Rubisco-limited photosynthesis and RuBP-regeneration-limited photosynthesis, and due to over-parameters for non-linear curve-fitting with g m included. We proposed an alternate solution to A/C i curve-fitting for estimates of V cmax , Rd, J max and g m with the various A/C i curve-fitting methods. The study indicated that down-regulation of photosynthetic capacity by elevated [CO 2 ] and leaf aging was due to partially the decrease in the maximum rates of carboxylation and partially the decrease in g m . Mesophyll conductance lowered photosynthetic capacity by 18% on average for the case of soybean plants.