For the case that the value of the maximum light use efficiency (MLUE) is not optimized for different steppes, we simulated the MLUE for meadow steppe, typical steppe and desert steppe in Inner Mongolia based on the field observed NPP and CASA ecosystem model, and analyzed the spatial and temporal pattern of the LUE and net primary productivity (NPP) in Inner Mongolia. The result indicate that the MLUE is optimized to be 0.654, 0.553 and 0.511 gC·MJ-1 for meadow steppe, typical steppe and desert steppe in Inner Mongolia, respectively, with an average of 0.573 gC·MJ-1. Compared to the result that used same value of 0.541 gC·MJ-1 for MLUE, the correlation coefficient and relative mean square error was improved 0.024 and 2.62 gC·(m2·month-1)-1, respectively after optimization. Affected by the hydrothermal condition and distribution of grassland types, the LUE and NPP in Inner Mongolia decreased from northeast to southwest, and showed one crest shape. However, the maximum value of LUE and NPP was appeared in August and July. This difference could be attributed to the difference in the maximum value between absorbed photosynthetically active radiation and LUE. The LUE and NPP decreased by meadow steppe, typical steppe and desert steppe.