Abstract: Objective: This study was designed to characterize the ultrastructure of the coccoid Helicobacter pylori(H. pylori) forms in the tissues of carcinoma.Methods: Transmission electron microscopy(TEM) technique was used to examine the structure of coccoid H. pylori in 4 cases of gastric carcinoma and 2 cases of esophageal carcinoma.Results: It was found that the coccoid H. pylori were scattered in cancer nest and cancer interspace,some of which enterded cancer cells,macrophages and other cells.The coccoid H. pylori appeared to be similar to the L-forms of other bacteria with varying cell shape and size,different cytoplasmic electron densities and defects in cell wall.Two types of coccoid H. pylori could be distinguished in the tissues of carcinoma by TEM.Type A was smaller with higher electron density in the cytoplasma,and flagellation on the cell membrane,suggesting that it could be viable.However,the type B was larger with lower electron density in the cytoplasma,and no flagellation on the cell membrane,suggesting that it could be a degenerative form.Conclusions: The coccoid forms of H.pulori still have certain degree of viability,thus they may play an important role in the transmission of H. pylori,the relapse of H. pylori related disease after treatment and the development of the related carcinoma.