Abstract: The BG140V/BG155V ultra-high strength/toughness casing is developed by means of studying the alloying and micro-alloying technologies，and optimizing the manufacturing process.The casing is in possession of excellent Charpy impact toughness and yield strength up to 1 000 MPa，and transverse Charpy impact value at 0℃surpassing the nominal yield strength by 10%，having been satisfactory to the requirements for the hi-strength/ toughness casing as serving the deep oil/gas well and shale gas drilling.The microstructure of the BG140V/BG155V ultra-high strength/toughness casing is characterized by means of optical microscopy（OM），scanning electron microscopy（SEM），electron probe microanalysis（EPMA）and electron backscattered diffraction（EBSD），etc.As a result，it is identified that the main elements responsible for the toughness enhancement of the casing are the improvement of chemical segregation，and refined precipitates and grains.