Nitrogen is most abundant element in atmosphere as well as it is widely distributed in lithosphere and biosphere. Nitrogen is very important and basic component of all living organisms. It is present in the forms of nucleic acid, proteins, amino acids, chlorophyll. it enters soil with manures, fertilizers and nitrogen fixation. There is various form of nitrogen in soil which is generally transformed by micro organisms. The transformation of nitrogen by microbes changes the nitrogen level in environment. This transformation has goes various steps and form a cycle, by this nitrogen is travel various states. Soil is enrich with nitrogen by its microbial mineralization of organic matter, which is necessary for producing crop yield. By cereals residues 15 to 45kg/ha and legumes residues are 80 to 144 kg/ha nitrogen mineralize in soil. Biological nitrogen fixation increase nitrogen in soil by 30-50 kg/ha/year. In recent decades mismanagement of fertilizer in soil has imbalance the nitrogen cycle. That why its study is more important. In this article we are going to explain how nitrogen is transformed and microbes are playing important role in this cycle.