Bacterial biomass is considered as one of the central parameters in the research on marine ecosystems. Now, the methods to convert bacterial biomass could depend on bacterial volume, but there is some difference among the different conversion models. Not only the measurements of bacterial volume and bacterial carbon concentration, the conversion factor between the volume and bacterial biomass, but also the conversion model will be discussed in this paper. Conclusions can be reached that flow cytometry is available to assess bacterial volume and X ray microanalysis is the best method to measure single bacterial biomass and allometric model is widely used in conversion of biomass.