Abstract: From July 2006 to May 2007, seasonal changes of fouling organisms on the cages were examined in the sea areas beside Xiushan Island and Changzhi Island through randomly sampling, identifying and measuring of the samples. The aims were to discover the monthly variation of fouling species, their biomass and density in the two sea areas. The results showed that the fouling organisms were composed of Hydra vulgaris, Actina sp., Caprella kroyeri, Gammarus sp., Balanus sp., Mytilus edulis, Ostrea sp., Ulva pertusa, and Enteromorpha sp. Their standing crop biomass and attaching density were recorded as well. The monthly variation was characterized that fouling organisms occurred heavily from July to November. After November, the amount of fouling organisms, such as Ulva pertusa, Anthopleura pacifica, reduced greatly with the temperature decreasing. From March of next year, the amount of fouling organisms, such as Caprella kroyeri, Anthopleura pacifica, Ampithoe valita, started to increase gradually with the temperature rising.