Abstract: The varied environmental conditions imposed by high frequency cycles of aquatic and aerial states is a unique problem confronted by the photosynthesis of intertidal macroalgae. The photosynthetic responses to light and temperature were examined in Ulva lactuca collected along the Shantou coast of China, to establish the photosynthetic characteristics in relation with tidal cycles for this common intertidal green macroalga. Under aerial state, the relationship of light_saturated net photosynthetic rates (Pmax) and exposure duration could be modeled by the third_order polynomial equation, and temperature affected the coefficients of the equation. An increase of photosynthetic activity was evident at 15% water loss, and the rates decreased thereafter with further desiccation till to zero at 80% water loss. Pmax was much more affected by temperature under aquatic state than under aerial state. Aerial Pmax (fully hydrated state) was significantly greater at 10℃, but was considerably lower at 30℃, compared with aquatic Pmax. The net carbon fixation under aerial state was greater than that under aquatic state within 6 h of photosynthetic duration at 10℃, whereas was slightly higher than that under aquatic state within 2.2 h at 20℃. At 30℃, the net carbon production was always lower under aerial state than under aquatic state. It was concluded that the photosynthesis and carbon budget of Ulva lactuca under aquatic state differed from that under aerial state, and such difference was associated with the conditions of ambient temperature and water statues of thalli.