Mesophyll conductance (gm) is the diffusional capacity of CO2 inside mesophyll cells. It was regarded as an infinite constant in research on photosynthetic process in the past. But results of relevant research in the past 20 years show that mesophyll conductance is not only a finite value but also changes with many environmental factors (temperature, water content, leaf nutrient content). Comparing to stomatal conductance (gs), mesophyll conductance is a more important limited factors to photosynthetic rate. More and more attentions have been paid on its role in studying plant eco-physiology and the relationship between global climate change and terrestrial ecosystem. Processes of discovery and development of mesophyll conductance and relevant research progress were reviewed systematically. The role of mesophyll conductance in CO2 diffusion process during photosynthesis was stressed. Tentative definition and value span of mesophyll conductance were summarized. This paper introduced theories and mathematical processes of methods frequently used for measuring mesophyll conductance, including instant carbon isotope discrimination method, electronic transport rate (J) variable method, electronic transport rate constant method, method of difference between intercellular CO2 and inside chloroplast CO2, and method of initial slope different between A-Ci and A-Cc curve. Advantage and disadvantage, as well as attention points, of the methods were deeply discussed. Responses of mesophyll conductance to environmental factors (temperature, water content, concentration of ambient CO2 and O3) were summarized. Relationship between mesophyll conductance and indicator of long term plant water use efficiency (WUE), δ13C of leaves, was analyzed and discussed. In addition, application of mesophyll conductance in interpreting ecological strategy of plants was also discussed. Perspectives for the future research on mesophyll conductance were suggested finally.