Samples were collected from paddy fields in Çorum- Osmancık area. Nitrogen- free BG-II medium was used for the isolation of nitrogen fixing cyanobacteria. Acetylene reduction technique was used to determine the effect of light intensity on the nitrogenase activities of the cyanobacteria, which were identified at the genus level. Moreover, in all genera while 600 lux light intensity was resulted in the highest nitrogenase activity, 900 lux light intensity was found to suppress the same activity more than 300 lux. As for the growth of all genera, biomass was stimulated at initial period but increasing light intensity was repressed the biomass.