Nitrogen fixation is a great challenge in solving the food supply of mankind. However N2 activation is extremely hard. Up until recently, the investigated catalysts for N2 fixation were based on metallic reducing agents. They are generally not environment friendly. We designed organocatalysts (carbenes) for nitrogen fixation using density functional theory (DFT) and ab initio theory. The reactivity of the carbene catalysts is mainly related to the electrostatic properties of the side chains. We compared the binding affinity to N2 with various carbenes (:CF2, :CCl2, :CBr2, and :CI2). We revealed that the electron donating ability of the central carbene carbon is the most important factor for trapping N2. Among heterocyclic carbenes, the cyclic diphospinocarbenes (PHC) represented a good candidate moiety for an efficient catalyst. We further designed the carbene based catalyst which has two carbene moieties to chelate N2 and investigated the whole catalytic mechanism. The highest energy barrier of the entire catalytic cycle is 28.5 kcal/mol, which is comparable to the previously reported metallic catalysts. This demonstrates the possibility of novel organic catalysts for nitrogen fixation.