We investigated the effect of light intensity on the correlation between optical density (OD) and cell mass concentration in high cell density culture of Anabaena variabilis. When the light intensity was gradually increased to maintain specific irradiation rate above 10 mmol/s/g dry cell, the dry cell mass concentration to OD ratio changed when the cell density became higher than OD 10, while the correlation was linear when light intensity remained constant after 2.5 days. When the OD was below 10, the dry cell concentration to OD ratio in unicells was the same as that of filaments, indicating that filament length, unicell size, and unicell dry cell concentration did not affect the dry cell concentration to OD ratio. Scanning electron microscope pictures revealed the differences in morphological structures between filaments below and above OD 10. Therefore, we propose that the morphological structures of filaments affected the dry cell concentration to OD ratio.