The impact of supplementation of Laminaria japonica powder on ruminal microbiota composition and fermentation parameters was evaluated in the present paper. For this purpose, a total of 30 cross bred Angus bulls (Xiangxi Yellow Cattle ♀ x Angus ♂, body weight = 331.22 ± 10.15 kg) were equally divided into two groups (n= 15). Two experimental diets were prepared that were basal diet (CON) and basal diet supplemented with 0.7% Laminaria japonica powder commonly known as kelp powder (SWE). The trial period was 60 days. The results showed that the addition of Laminaria japonica powder to the diet had no effect on rumen fermentation parameters in bulls (P>0.05). Results of microbial compositions explored that at both phylum and genus levels, most abundant phylum was Bacteroidetes (59.82%) and Firmicutes (29.75%). Results also showed that most abundant taxa were Prevotella (35.04%) and Ruminococcus (7.52%), and at the genus level 37.68% of the sequences were unclassified. The results of the relative abundance of the top 10 genera of rumen showed that Succiniclasticum, Moryella, and Comamonas were significantly different in ruminal bacteria relative abundance between the two experimental treatments (P<0.05). There was a notable distinction between CON and SWE cohorts using PC2 (P<0.01, 16.66%). Based on results, it is concluded that Laminaria japonica powder influence rumen microbial composition.