The pyrolysis is one of the promising methods to convert biomass to bio-oils as fuels and chemicals. During the pyrolysis of biomass, some nitrogen species are released, as the precursors of NO_x with potential environment pollution. The release characterization of nitrogen species during biomass pyrolysis is reviewed in this paper. Firstly, the content and the distribution of nitrogen in various biomass materials are classified, and the research methods, the conclusions and the problems that remain to be solved concerning the chemical forms or the nitrogen functionalities in different biomass materials are discussed. And then we discuss the release characterization, as well as the factors affecting the nitrogen evolution during pyrolysis. The evolution route of nitrogen in biomass during pyrolysis is elaborated in detail, and the operating parameters such as the temperature, the heating rates and the nitrogen forms and content are shown to contribute to the nitrogen release., The nitrogen distribution in products in gas phase, liquid phase and solid phase is analyzed as well. In the end, some prospective developments are commented for the research of pyrolysis convention of biomass.