Climate change will affect various sectors such as agriculture, water resources, etc. Decision-making on an emission reduction pathway of greenhouse gases (GHG) and a climate stabilization target is the issue to be solved through international cooperation. To support the decision-making of impact assessments on multiple sectors, an integrated analysis tool named AIM/Impact[Policy] was developed. In AIM/Impact[Policy], impacts of climate change on various sectors under assumed GHG-emission pathways are projected based on impact functions. An impact function is a look-up table of the country-averaged impacts on a certain sector under arbitrary conditions of climate change. In this study, we developed impact functions for maize, wheat, and paddy-rice, which will be used in AIM/Impact[Policy]. We conducted a sensitivity analysis using a full-scale impact model, named M-GAEZ model, with varying temperature and precipitation. We averaged the crop yield for each country to obtain the impact functions. According to the analysis, the sensitivity of crop yield to the temperature and the precipitation differs by both crops and countries. Among top producing countries, the maize yield in Brazil and the paddy-rice yield in Indonesia decrease in most cases. In India, the wheat yield decreases to around 30 % of the present when the temperature increases by 4 °C, that is, wheat hardly grows under such conditions. The impact functions enable us to derive the impacts on crop yield in each country under various assumptions of climate change (i.e., the changes in temperature and the changes in precipitation), and they are useful for supporting policy decisions.