Efficient use of energy resources in crop production is an important goal in sustainable agriculture. This study compares the energy flow in farming systems across farm size with their corresponding greenhouse gas (GHG) emissions - presented in terms of carbon dioxide equivalent (CO2 eq.) - in the north of Iran. To reach this aim, primary data were collected by survey with farmers whose main activity was major crops production in the region that included wheat, barley, canola, soybean, paddy and corn silage. The results showed that total energy input for corn silage (52.1 GJ ha(-1)) is greater than other systems. The results also revealed that yield and output energy of crops were not significantly affected by field size, whereas energy use efficiency of systems increased significantly as field size increased. Study shows that the cultivation of paddy emits the highest CO2 eq. emission (6094 kg CO2 eq. ha(-1)) among crops, in which around 60% is contributed by methane (CH4). The efficient use of fertilizers, optimized pumping facilities for irrigation, stopping of crop residue burning in the field and use them for energy supply could be among the options to improve energy use efficiency and mitigate GHG emissions. (C) 2013 Elsevier Ltd. All rights reserved.