The fattening has a substantial impact on production performance, nutrient digestion, and me-tabolism in Angus beef cattles. This study investigates the effects of varying levels of concentrate diet on the performance of Angus beef cattles during the growth (7 to 12 months of age) and fin-ishing (13 to 30 months of age) phases. A consecutive feeding trial was conducted on a uniform cohort of fattened Angus steers, categorized into two distinct phases based on body weight and concentrate feed levels: the Low-concentrate fattening period (L,growing period) and the High-concentrate fattening period (H,fattening period). The trial spanned 9 weeks for each phase, dur-ing which the performance of the steers in these two feeding periods was systematically investi-gated and compared. Rumen fluid, blood, and feces were collected at the conclusion of the pre-fattening and post-fattening periods, allowing for a comparison of performance, rumen microbio-ta, fermentation parameters, serum biochemical indexes, and nutrient apparent digestibility. This provided insights into the changes in performance and nutrient digestion and metabolism of beef cattle at different stage of production.