A variety of diseases caused by Rickettsia bacteria like - RLB has occurred in crustaceans such as crabs, prawns and made significant economy lose. Particularly RLB-caused disease seriously occurred in spiny lobster farms in 2007 in Vietnam and continuous every year. This study presents the results of morphologic characteristics and 16S rRNA gene sequence of RLB infected spiny lobster (Panulirus ornatus) with milky disease in Vietnam. The causative agent of the disease was believed to be a gram-negative bacterium including RLB that could not be cultured on a range of agar-based media. Morphologic studies by TEM, SEM and the sequence analysis of 16S rRNA gene from RLB have indicated that they are a previously undescribed species of a-proteobacteria and belong to Rickettsia. The 16S rRNA gene sequences have been registered on Genbank database with the accession number HQ130336 and HQ130337. The results of the comparison between the above two sequences and some other sequences of C. maenas, P. monodon and Panilurus spp on GenBank database showed the highly similar sequence.