Abstract: Fracture toughness KIQ of high strength X140 casing steel sample with three different thicknesses was measured according to the national standard GB/T 4161-2007. Based on the theoretical formulas on the relation between fracture toughness and sample thickness, the material constants ξ and κ were got by using least square method. A new analytical formula of fracture toughness, which was related to the sample thickness, was proposed, and the values of KIC of different thickness samples were calculated. J resistance curve test was conducted to verify the accuracy of the new formula. The results show that, for X140 steel, calculated fracture toughness agreed well with the result from J test. With the increase of the thickness,fracture toughness increased at first, then decreased and stabilized finally.