Elevational diversity patterns for macroorganisms have been studied since the advent of biogeography but still very little is known of how the microbial diversity in soils varies with elevation. We chose Mt. Halla of Jeju Island, South Korea, to study how microbial diversity varies in a geologically and topographically simple mountain system. Samples were taken along two different transects at elevational intervals and PCR amplified soil DNA for the bacterial 16S rRNA gene targeting V1 to V3 region was pyrosequenced and taxonomically classified with reference to a bioinformatic database. The two transects, on trachybasalt and basalt respectively, showed striking consistent differences in bacteria phyla composition which must relate to some aspect of the soil chemistry or texture. Elevation was found to be significantly correlated with bacterial diversity and richness, and was the best overall predictor of variation. There was a 'dip' in diversity lower mid elevations around 700-1300m at both transects, which is quite different from trends found in previous elevational studies in other parts of the world. Soil and climatic parameters that explained a significant part of the variation in OTU diversity were pH, soil phosphorus, MAT and MAP. When the mosaic of vegetation cover was taken into account, significant differences in bacterial diversity were found between different vegetation types. The analysis shows that two main factors affect bacterial community structure on Mt Halla: Climate- MAT & MAP and Soil composition- pH and phosphate concentration which were significantly correlated with the diversity and richness throughout the elevation on both transects. Vegetation cover is also consistently associated with its own levels of bacterial diversity, but its relationship to pH and to geology makes it difficult to disentangle as a cause or incidental correlate of the changes in bacterial diversity.