The engineering fracture theory of void growth ratio (VG theory) was proposed by Zheng et alin 1983<2>, but up to now the limits of its applicability, which include the limits on torsional prestrainand low temperature, arc not clearly defined.By the mechanical test and SEM measurement conducted on the smooth cylindrical sl)ccimensof Chinese 20 steel (approximately equivalent to ASTM 1020 sted) the new fracture toughnessVGC and corresponding microform parameter of critical void growth ratio RC/ RO are determinedrespectively. After analyzing the experimental data of VGC and RC/RO, the authors find that theVG engineering fracture theory is applicable when torsional prcstrain is less than 1.2 or temperatureis not lower than -60℃. In other words, the coefficient C in the relationship VGC= Cln(RC/ RO) is amaterial constant.When the torsional prestrain is groatcr than 1.2 or when tcmpcraturc is lower than-60℃,the coeffiaient C is no longer a constant but varies irregularly (see Figs.4 and 6).The authors' study on the effect of torsional prestrain also reveals two findings:1. Torsional prestrain does not appear to cause the nucleation and growth of voids in Chinese 20steel.2, Torsional prestrain can cause ductile-britUe transition in the tensile fracture of Chinese 20sted. The nil ductility torsional prestrain is about 1.75. When the prestrain is up to 1.75, the microscopic mechanism of tensile fracture becomes complete cleavage (see Figs.3 and 5).