Gracilaria tenuistipitata is an edible red alga and can be utilized as feed for small abalone and foodstuff (ex. agar extract). It is the major species of Gracilaria that is commonly cultivated and consumed in Taiwan. Effects of environmental factors on prostaglandin E2 (PGE2) production in the seaweed G. tenuistipitata were investigated in this study. PGE2 amount increased nearly 2-fold and 47% when the seaweed was exposed to two ways of fluctuation of seawater salinity from 2% to 1% and 3% for 12 hr, respectively. The stresses of low temperature and irradiance promoted the seaweed to produce PGE2. When the seaweeds were exposed to air for 2 hr and 4 hr, the PGE2 levels were elevated by 25% and 31%, respectively. When the seaweed was soaked in freshwater, PGE2 amount gradually decreased. Cu2+ and Zn2+ inhibited PGE2 production at the concentrations of 3 and 50 mg/L, respectively. Mg2+ slightly inhibited PGE2 production about 20% at 2,025 mg/L, while Ca2+ boosted PGE2 production about 59% at 600 mg/L. Hence, environmental factors significantly affect the PGE2 production in the seaweed G. tenuistipitata. Prostaglandins might be associated with processes permitting the algae tissue to survive in unfavorable conditions. To lower the amount of PGE2 in the seaweed and ensure the food safety, it is suggested to minimize the variation of growth condition to avoid PGE2 increase in the seaweed during cultivation period. In addition, consumers should avoid eating raw seaweed since PGE2 could be destroyed by heating.