Twelve typical mixed rations of sheep were formulated to study the relationship between in vitro gas production (GP) and in vivo nutrient digestibility (D). The in vitro GP of the rations at, h: 2, 4, 6, 12, 24, 48 and 72 was determined using the Hohenheim gas test. The in vivo digestibility of dry matter (DM), organic matter (OM) was determined using three adult sheep in four 3×3 Latin square design experiment in an earlier study (Xue et al., 2006). The GP at different incubation time was fitted to the model GP = a+b (1–e^-ct) (Ørskov and McDonald, 1979) and the GP parameters a (ml/200 mg DM), b (ml/200 mg DM), a+b (ml/200 mg DM) and c (%/h) were calculated. It was found that there were significant regression relationships between DM and OM digestibility and GP (P<0.001), and between DM and OM digestibility and GP parameters a, b and c (P<0.01; P<0.001)