The quality of the rice straw silage added with apple pomace was investigated in this study and the amount of apple pomace added in different treatments were 0, 20, 40 and 60%, respectively. Crude protein contents (6.4-7.5%) of rice straw silage added with apple pomace were significantly (P<0.05) higher than that of 100%. rice straw silage (5.3%), however, crude ash contents were lower (P<0.05) in supplementation of apple pomace. The trends of changing chemical composition between raw materials and end products of silages particularly in the contents of crude protein and crude ash were more apparent in the silages added with apple pomace by 40-60%. Values of pH and the contents of lactic acid and total acid in 40-60% apple pomace added silages were 3.9-4.1, 1.0- 1.5% and 2.7-4.5%, respectively which were significantly (P<0.05) higher than those of 4.6, 0.02% and 0.34% in 100% rice straw silage, respectively. In situ dry matter (DM) and neutral detergent fiber (NDF) disappearance rates in the rumen in the treatments of 40- 60% apple pomace added silages were significantly (P<0.05) higher than those of 100% rice straw silage particularly since after 3 and 24 hour incubation on DM and NDF disappearance, respectively. Although quickly degraded fraction (a) among the treatments were not significantly different, 28.4-28.5% of slowly degraded fraction (b) and 27.2-27.4% of effective degradability (ED, k=0.08) for DM were significantly (P<0.05) higher than those of 100% rice straw silage (12.5 and 24.6% respectively). NDF was in the same trend as in DM. 31.6-63.2% of NDF for b fraction and 18.7- 19.4% for ED in 40-60% apple pomace added silages were significantly (P<0.05) higher than those of 100% rice straw silage (12.4 and 17.6% respectively). (Key words : Rice straw silage, Apple pomace, Lactic acid, In situ digestibility, Effective degradability)