In the natural aquatic environment, there are various types of biogenic manganese oxide (BMO) precipitates which are produced by manganese oxidizing microorganisms, at ambient temperature, atmosphere and neutral pH.BMO is a very interesting and useful functional material because of its unique property; hollow globule shape formed by nanosheets, large surface area and photocatalytic characteristics. In this study, thermally-processed BMO at 100 − 1000 °C was analyzed by XRD, XANES, SEM, TEM and nitrogen adsorption method to reveal the effects of heat-treatment on its crystal structure, Mn valence state, globule morphology, microstructure, pore size distribution and specific surface area. In the result, morphology of BMO is maintained below 800 °C, while nanosheets composing BMO were changed into nanoparticles at 600 °C. The crystal structure was transformed from birnessite into hausmannite (Mn₃O₄) at 600 °C. The specific surface area was increased by heat-treatment between 300 and 500 °C. In cases of artificial manganese oxides, such changes of crystal structure and specific surface area were not observed, and combustion of organic materials produced by microorganisms in BMO probably has a large effect on them.