[Background] Cyanobacteria are key contributors to biological nitrogen fixation in aquatic and terrestrial ecosystems. [Objective] To increase our knowledge of nitrogen-fixing cyanobacteria in paddy soil and obtain culturable nitrogen-fixing cyanobacterial strains for further studies. [Methods] Three paddy soils with different nitrogen fixation capabilities were selected to isolate nitrogen-fixing cyanobacterial strains with BG11-N medium. The morphological characteristics of newly isolated strains were observed. The nifH gene amplification of genomic DNA was used to clarify their nitrogen fixation potential, acetylene reduction assay and ¹⁵N₂ tracer method were used to quantitatively determine their nitrogen fixation activity. Preliminary identification of the newly isolated strains were determined through 16S rRNA gene sequence comparison. [Results] Seven autotrophic strains were isolated and purified with BG11-N medium under light conditions. All cells were round or elliptical, uniseriate, unbranched, filamentous and moniliform, and formed mat-like colonies on solid medium. The newly isolated strains grew well in BG11-N medium, were positive for nifH gene amplification of genomic DNA, had high nitrogen fixation activity measured by acetylene reduction assay and ¹⁵N₂ tracer method, and had capabilities of siderophore production. Through 16S rRNA gene sequence comparison and morphological features, all the seven strains could be preliminarily identified as belonging to Nostocaceae. [Conclusion] The cyanobacterial (Nostocaceae) strains that plays important role in biological nitrogen fixation were isolated from paddy soil. The culturable nitrogen-fixing cyanobacterial strains have high nitrogen fixation and siderophore production ability, which could be used as microbial resources for further studies and have potential research and application value.