With the aid of the DeNitrification–DeComposition (DNDC) model，the total CH₄ emission from of paddy fields in the period from 1982 to 2000 in the Taihu Lake region，as was affected by grid size，was estimated and variation of the total emission with grid size characterized A total of 7 grid sizes，i.e.1 km，2 km，4 km，8 km，16 km，32 km and 64 km，were designed，based on the 1∶50 000 soil database. Results showed that an apparent turning point was observed with the grid size of 16 km，and displayed as a limited range of the estimation of the total CH₄ emission being only 0.27 Tg C and low variation of the simulation of grid sizes from 1 km to 16 km.However，the range of the estimation expanded to 2.96 Tg C due to errors in area calculation in grid sizes from 16 km to 64 km，and the variation increased rapidly. Taking into account the amount of spatial data and computing cycle of the model，16 km was the optimal grid size for estimating emission of greenhouse gas from the farmlands.