Abstract: The tests of daily variation of photosynthetic rate of butterhead lettuce grown in green house are conducted. The results show that the daily variation of photosynthetic rate is a two peak curve in sunny days in summer and autumn season. While in winter with lower temperature, it is a
one-peak curve pattern. In the cloudy and raining days, the change of photosynthesis just follow the change of radiat ion. The photosynthesis maintaining at its high value is not more than 3- 4 hours within a day. If the radiation and temperature cannot meet the need of crop in the morning, it is still possible to produce a photosynthet ic peak in the afternoon with suitable environment factors for the crop photosynthesis.

Key words: Lettuce, Butterhead lettuce, Photosynthesis, Daily variation, Greenhouse