The relationship between microstructures and mechanical properties of a medium carbon V-N microalloyed steel used for N80 seamless oil-well tubes of hot rolling non-quenched/tempered (non-Q and T) was investigated.The results have shown that volume percentages of upper bainite,modified bainite and ferrite have a decisive influence on impact energies of steel tubes.When the total volume percentage of bainite is larger than 5%,the impact energy of tubes can not satisfy with the industrial criteria.Moreover,if the total volume percentage of bainite is smaller than 5%,then the impact energy of steel tubes enhances with volume percentage of ferrite increasing.The final microstructures have closely relation with tube billet quality,controlled cooling temperature after tube rolling and cooling method after stretch-reduction-diameter.High quality of medium carbon V-N microalloyed steel for non-Q and T oil-well tubes can be produced through comprehensive control of microstructures and mechanical properties in sub-procedures,especially for tube billet quality and controlled cooling parameters. The relationship between microstructures and mechanical properties of a medium carbon VN microalloyed steel used for N80 seamless oil-well tubes of hot rolling non-quenched / tempered (non-Q and T) was investigated. The results have shown that volume percentages of upper bainite , modified bainite and ferrite have a decisive influence on impact energies of steel tubes. The total volume percentage of bainite is larger than 5%, the impact energy of tubes can not satisfy with the industrial criteria. More than, the the total volume percentage of bainite is smaller than 5%, then the impact energy of steel tubes enhances with volume percentage of ferrite increasing. the final microstructures have closely relation with tube billet quality, controlled cooling temperature after tube rolling and cooling method after stretch-reduction-diameter. High quality of medium carbon VN microalloyed steel for non-Q and T oil-well tubes can be produced through comprehensive control of microstructures and mechanica l properties in sub-procedures, especially for tube billet quality and controlled cooling parameters.