We investigated the changes in photosynthetic activity, L* value and chlorophyll fluorescence of Pyropia yezoensis thallus under low nutrient condition. Photosynthetic activities decreased when L* values of thallus were the higher values of 65. As for R-SD values extracted from chlorophyll fluorescence using graphic software, photosynthetic activities and relative values of R-SD decreased when thallus were exposed to low nutrient condition. Under low nutrient condition, it was suggested that thallus was maintained for photosynthetic activity using resources of nitrogen, phosphorus, iron and so on within cells. While, by lacking of their resources, it was thought that thallus was not able to produce photosynthetic pigment and photosynthetic activity was decreased. In the present results, chlorophyll fluorescence has a potential to become useful tool for finding the low nutrient stress of P. yezoensis thallus.