Tensile properties and fracture toughness of AISI 4340 rolled steel have been measured as a function of test temperature, tempering temperature (-100℃ to 200℃) and tested direction, also fractographic observation was made by means of scanning electron microscope to find out the property difference between rolling and transverse directions, and ductile and brittle fracture toughness behavior. The tensile and yield strength decreased with increasing tempering temperature and increasing test temperature. while the specimens tempered at 550℃ and 650℃ showed higher elongation value at low temperature (-50℃ and -100℃) than at room temperature (25℃). The relation between KIC2/E and CVN values varried wih test direction and test temperature. The fracture toughness G1C values in rolling direction were in the range between l.5×l0(4) and 3.9×l0(4) J/m2 when the stretch zone size in front of fatigue pre-crack in compact tension specimens decreased with decreasing test temperature.