Abstract: The flow unit is defined as the same reservoir unit with identical seepage features, and it is considered that the flow unit is relative and in some level and scale. After analysing the basic features of flow units, a genetic classification scheme of flow units was suggested. Flow units were classified as seven types controlled by faults, impermeable strata, intercalated bads, permeability rhythm, stratification tectonics, fissures and pore structure, respectively. The flow unit of different genesis reflects the scale and level of reservoir heterogeneity and has different studying contents and methods.