When the circumferential scanning for thickwalled pipe is carried out with ultrasonic time of flight diffraction (TOFD), the ray path of lateral wave is inconsistent with the curved surface, which leads to the quantitative errors of length and angle for inclined cracks. To achieve deep crack detection, the probe center spacing (PCS) may be increased, which further increases the measurement errors. In this paper, considering the geometrical relationship among the curvature radius of pipe outer wall, PCS and crack tip depth, the accurate quantification study of the inclined cracks in thickwalled pipes is conducted. The quantitative formulas for the crack length and inclined angle are deduced for the detection of thickwalled pipes with circumferential scanning of TOFD, and the measurement errors before and after optimization are compared. The simulation results show that the quantitative errors of crack length and inclined angle are reduced by 010 mm and 158°, respectively for the crack with crack length of 40 mm and inclined angle of 10°~50° in the carbon steel pipe with wall thickness of 300 mm and curvature radius of outer wall of 1480 mm. Experiment on a carbon steel pipe specimen was conducted. In the experiment, the quantitative errors of crack length and inclined angle are reduced from 030 mm and 274° down to 027 mm and 028°, respectively for the crack with crack length of 40 mm and inclined angle of 30°. The proposed method is suitable for the quantitative detection of inclined cracks in pipes with different curvature radii, and has wide application fields.