In the present research the growth and metabolism of cyanobacterium Fischerella sp. FS18, isolated from rice fields of Gillan Province (Iran), were evaluated in different nitrogen sources. This strain is a good candidate as biological fertilizer due to high efficiency to N2 fixation. In this way, cells in logarithmic phase of stock were treated with different nitrogen sources (N-free, 1 & 2 mM NH4+ and 5 and 10 mM NO3?). Results indicated that maximum growth rate belonged to NO3? treatment. Dry weights of Fischerella sp. FS18 in N-free medium and ammonium were in decreasing order of preference; and NH4+ 2 mM was drastically toxic for this strain. The highest chlorophyll concentration was detected in N-free medium. NO3? 5 mM and NH4+ 2 mM had the lower contents of this pigment. Photosynthesis rate varied the same as chlorophyll. phycocyanin and phycoerythrin had the highest rate in nitrate. The highest light-saturated photosynthetic rate and also steepest initial slope clearly belonged to nitrogen free-medium. All types of combined nitrogen sources had inhibitory effect on nitrogenase activity, regardless of the concentrations used. Our results indicated that Fischerella sp. FS18 is a potent diazotrophic species with increasing growth in the presence of nitrate; and ammonium 2mM seems to be toxic for it.