Fractures constitute one of reservoir storage spaces and control hydrocarbon accumulations and well productions of the buried carbonate hills of Ordovician Majiagou Formation in Nanpu sag offshore, Bohai Bay basin. Data of imaging logging, core description and outcrop investigation were adopted to probe fracture lengths, densities, openness, fillings and orientations within the buried carbonate hills in Nanpu sag offshore and discuss controls on fracture growth and growth mechanisms of fractures. The fractures are mainly made up of tectonic fractures dominated by fracture apertures of 5 to 50μm for the most part and have a relatively extensive distribution. Controls on the fractures principally are rifting and paleo-weathering. The former leaded to tectonic fractures corresponding with the normal faults, and the latter caused nontectonic fractures, including weathering fractures, bedding fractures and dissolution and collapse fractures that are almost entirely distributed in the vadose zone on top of the buried carbonate hills. The growth of tectonic fractures underwent three periods, including compressing before the Eocene, rifting from the stage of Eocene Shahejie to Miocene Dongying, and striking-slipping since the stage of Miocene Guantao. Effective fractures for hydrocarbon accumulation of the buried carbonate hills in Nanpu sag offshore were mainly created during the process of rifting from the stage of Eocene Shahejie to Miocene Dongying.