Abstract: This study used an in vitro gas production technique to investigate the associative effects of wheat straw (WS) and rice bran meal (RBM), on the fermentation characteristics of WS mixed with RBM at rations of 100:0 (RBM0), 75:25 (RBM25), 50:50 (RBM50), 25:75 (RBM75) and 0:100 (RBM100). Key results were:1) For gas measurements, 12, 24 and 48 h gas production (GP), maximum theoretical (HM) and rate of biogas production (B) increased while the lag time reduced incrementally (P<0.05) with the percentage of RBM added; 2) The RBM25 group had the highest positive associative effects for B (P<0.05); 3) For dry matter degradation (DMD), neutral detergent fiber degradation (NDFD) and acid detergent fiber degradation (ADFD),the RBM75 group had the highest values (P<0.05), while for DMD, the RBM25 group had the highest positive associative effects (P<0.05). For NDFD and ADFD, the RBM75 group had the highest positive associative effects (P<0.05); 4) With the RBM75 mixture, the acetate, propionate, butyrate and total volatile fatty acid production were highest, and acetate:propionate was the lowest; 5) For the 48 h GP, B was positively related to dry matter (DM), crude protein (CP), crude ash (Ash) and neutral detergent soluble (NDS) (P<0.01), but was negatively related to neutral detergent fiber (NDF), acid detergent fiber (ADF), organic matter (OM), hemicellulose (HC) and NDS/CP (P<0.01); HM was positively related to DM, CP, Ash, NDS (P<0.05), but it was negatively related to NDF, ADF, OM and HC (P<0.05); the lag was negatively related to NDS/CP (P<0.01). It is concluded that for B and DMD, the RBM25 mixture gave the best associative effects in this experiment. For fiber degradation rate, a RBM75 mixture gave the best associative effects.