Light-use efficiency (LUE) is an important parameter to assess light energy absorption of leaf. Especially, it is a key factor to affect production and quality of ecosystem. A model of LUE was developed based on a mechanistic model of lightresponse of photosynthesis. The maximum LUE (LUE_max) and corresponding saturation irradiance (I_L-sat) were deduced according to the LUE model. At CO₂ concentrations of 350, 450, 550 and 650 μmol·mol^-1, the lightresponse curves of LUE of tomato seedling leaves were simulated. The results showed that the model of LUE described well the response curves of light use efficiency of tomato seedling leaves at four CO₂ concentrations. LUE of tomato seedling leaves reached the maximum value at photosynthetically active radiation between 70-90 μmol·m^-2·s^-1. There were no difference of LUE_max and I_L-sat at 550 and 650 μmol·mol^-1. Regarding this phenomenon, it was hypothesized that the photosynthetic functions of tomato seedling leaves had acclimated to the low irradiance in greenhouse so that the intrinsic characteristic of lightharvesting pigments such as the effective light absorption crosssection of lightharvesting pigments and ratio of pigment molecules in the excited state to ground state had hardly changed at high CO₂ concentrations.