Objectives:The purpose of this study is to provide basic data for the development of a rapid and easy to measure alternative indicator for estimating the number of cyanobacteria cells in the field.Methods:Phycocyanin, algal cell number, and water quality items were measured at the left, middle, and right points of Maery Bridge near the Mulgeum and Maery algae monitoring network located downstream of the Nakdong River. The phycocyanin concentration(PC) was measured in the field using the LISST-HAB sensor, which is a fluorescence concentration analyzer, and comparative analysis was performed with the number of algal cells by algae species.Results and Discussion:As a limiting factor for blue-green algae occurrence, the correlation coefficient (r) between T-P and Blue-green algae (BGA) was 0.58 (≥0.3). Phycocyanin and Blue-green algae showed a high significant correlation at 0.96 (≥0.3). Prob (F-statistics) of the blue-green algae cell count estimation formula using phycocyanin was less than 0.05, showing a significant result. The estimation formula was estimated BGA(cells/mL) = 326.95×PC+50,484. As a result of the analysis of the predicted and measured values of Blue-green algae, r<sup>2</sup> was 0.988, indicating a high coefficient of determination and 92.5% accuracy.Conclusion: The correlation analysis with the number of cyanobacteria showed a high significant correlation with phycocyanin of 0.96 (≥0.3), followed by T-P, a limiting factor for the occurrence of cyanobacteria, of 0.58 (≥0.3) indicating a significant correlation. In addition, the formula for estimating the number of blue-green algae cells using phycocyanin was found to be BGA (cells/mL)=326.95×PC+50,484 in which r<sup>2</sup> was 0.927, and Prob (F-statistics) was less than 0.05. As a result of analyzing the predicted and measured values of cyanobacteria cell numbers, r<sup>2</sup> was 0.988, indicating a high coefficient of determination, with accuracy of 92.5%. Through this results, it reveals that the number of cyanobacteria cells can be estimated using the sensor of the fluorescence concentration analyzer at the target point by substituting the phycocyanin concentration into the blue-green algae cell number estimation formula.