Abstract: Kelp (Laminaria japonica) is important economic farming seaweed in our country, rich in vitamins, protein, minerals, fiber, inorganic salts and other substances. But with the continuous expansion of seaweed farming, rotten diseases that occurred during the breeding often cause significant economic losses. Rotten diseases are complex problems, relative researches have been carried out for many years, but yet there has not been a unified conclusion. Previously mainly method was to search for a single pathogen and develop corresponding inhibitors. With the theory of phycosphere and phycosphere microbes, the balance of phycosphere micro-environment was found important to the normal growth of algae, making it possible to study rotten diseases from the way of microbial diversity. In this study, marine bacteria were isolated from health kelps (Laminaria japonica), rotten diseased kelps and seawater in coastal area of Taizhou, China. Antibacterial activities were screened using agar diffusion method, and the relationship of active bacteria and their origin was studied. The results showed that 42 strains among total 143 isolated marine bacteria had antimicrobial activity, which indicated that marine bacteria in phycosphere were extremely rich in anti-bacterial resources. The proportion of active bacteria associated with health kelps and seawater (35% and 29%, respectively) was higher than that isolated from rotten diseased kelps (23%). Molecular phylogenetic analysis of marine bacteria with broad- spectrum antibacterial activities based on 16S rRNA sequences indicated that they belonged to the genera of Pseudoalteromonas, Rahnella, Donghaeana, Bacillus and Exiguobacterium, respectively. These results proved that marine bacteria in phycosphere had great diversity in class and quantity, while different inhibitory effect to seven sensitive indicator bacteria meant there was also diversity in function. Diversity condition changed as the growth state of kelp changed. This study provides a clue to understand the mechanism of rotten disease from microbial diversity, and provide some new materials with medical potential.