In algal research, oxygen evolution is often used as a direct measure of photosynthetic rate. Due to the facility and time limitation, however, direct measurement of oxygen evolution by algae sometimes is impracticable and inconvenient, and researchers therefore are often inclined to take electron transport rate (ETR) as an indirect measure of photosynthetic rate. Theoretically, in photosynthesis, the oxidization of two molecules of H2O leads to the production of one molecule of oxygen and four electrons; the electrons are ultimately transported to NADP+ and used to form NADPH, and oxygen is released to the atmosphere. In theory, there exists a linear relation of 0.25 molar ratio between oxygen evolution and electron transport. In algae, oxygen is involved as electron acceptor in many physiological processes such as chloro-respiration, water-water cycle, and photorespiration, which consequently affects the ultimate oxygen evolution in photosynthesis. Therefore, some researches have been conducted to ascertain whether oxygen evolution is linearly related with electron transport. It was found that if irradiance was saturated or dissolved inorganic carbon concentrations was changed, oxygen evolution did not present a linear relation with electron transport. However, under the condition of constant temperature, it was found that oxygen evolution was linearly related to electron transport. Because temperature affects many physiological processes in which oxygen is consumed (such as photorespiration), it is likely that oxygen evolution is not linearly related to electron transport as temperature changes. To test this hypothesis, three algal species, Chorella pyrenoidosa, Nitzschia sp. and Synechocystis aquetilis, belonging to chlorophyta, bacillariophyta, and cyanophyta, respectively, were selected in the experiment. Their electron transport rate and oxygen evolution under variable temperature conditions (10，15，20，25℃) were investigated, and the ratio of PGrosshotosynthesis (PGross, in terms of oxygen evolution) to electron transport rate was analyzed. The results showed that, for all three species, the ratio of PGross/ETR decreased significantly as temperature increased. At low temperature, the ratio of PGross/ETR was higher, which implies that more energy via electron transport might be used for photosynthesis and more oxygen was produced. On the contrary, at high temperature, the ratio of PGross/ETR was lower, which implies that presumably less energy via ETR was used to drive carbon fixation and oxygen evolution was reduced due to probable oxygen utilization in processes such as photorespiration. It is clear in this experiment that the relation between oxygen evolution rate and ETR was not linear, and was strikingly affected by temperature. Based on the experimental results, it is suggested that ETR measurements cannot be used to reliably estimate photosynthetic production under variable temperatures.