Abstract: The aim of this study was to determine the effects of adding the cellulose (CEL) and Lactobacillus casei (LC) separately or together on the quality of Leymus chinensis silage. Chopped L. chinensis was supplemented with 1060 U/g cellulose, 1×10⁵ CFU/g fresh matter L. casei, or both (LC+CEL). Chopped L. chinensis with no additives served as the control. Three replicates of each treatment were weighed and chopped into 0.5 L plastic buckets, and these mini silos were stored at ambient temperature for 45 days. The ultrastructure of stems, fermentation quality, and nutrient composition of the L. chinensis silage were analyzed. The pH in the LC+CEL mixture was 3.86, which was significantly lower than those in the LC and the CEL treatments (P<0.05), but the lactic acid and acetic acid contents in the LC+CEL mixture were significantly higher than those in the LC and the CEL treatments (P<0.05). The LC+CEL mixture showed significantly decreased neutral detergent fiber (43.9 g/kg), acid detergent fiber (22.3 g/kg), cellulose (28.5 g/kg), and hemicellulose (21.6 g/kg) contents, and increased water soluble carbohydrates (20.5 g/kg) and lactic acid (29.2 g/kg) contents after 45 days of fermentation. Compared with the control (no additives), all of the treatments showed significantly lower neutral detergent fiber and acid detergent fiber contents, and significantly higher water soluble carbohydrates and organic acids contents (P<0.05). Overall, the LC+CEL mixture performed better in terms of degrading fiber to water soluble carbohydrates and organic acids than did either LC or CEL, but all of the treatments performed better than the control. The biological additives degraded the different tissues to varying degrees, with greater degradation of parenchyma tissue and less degradation of lignified tissue.