The relation between light intensity and photosynthesis activity of four mainspecies of Gracilaria (i. e. G. tenusitipitata, G. sp.-Hainan. G. verrucosa,and G.sp.--Haifeng), in Guangdong province, were measured in May and December of 1983and in February of 1984. The results of the experiment as in the following:1. In the early summer, the common light compensation point of the four speciesof Gracilaria is 1,000 Lux. From December to February of the second year. while thelight saturation point of G.verrucosa and G. tenunistipitata is 12,000 Lux and 14,000Lux respectively. The difference between two seasons is so wide. That is because fromApril, to June it is the time for spores scattering and germinating while in winter itis the best time for growing. So that need for ligh intensity was different in differentgrowing season.2. The net photosynthetic rate of Gracilaria was comparatively low in the earlysummer,but was very high in December and February of second year.3. The net photosynthetic rate of the sub--branches were higher than that of themain branches. This characteristic was found in G. sp-Hainan in early summer 1983,and in G. tenuistioitata and G. verrucosa in February 1984.4. The net photosynthotic rate on the apical part of the sporophyte of G. tenuisti-pitata was higher than that on the middle and lower parts of which show the samerate. The results will provide a scientific basis for "cutting for increasing production"technique of Gracilaria.From the experiment, the results could be concluded: In expecting to get a goodharvest, the culturing-raft should be set in the upper layer of seawater during theirfast growing period so that the algae can receive strong light to strengthen photosyn-thesis. while the algae grow to late stage the raft should be set in the lower layer inorder to avoid the strong light and to minimize the adherence of miscellaneous algae.