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A rumen simulation technique apparatus with eight 800 mL fermentation vessels was used to investigate the effects of rumen degradable protein (RDP) level and non-fibre carbohydrate (NFC) type on ruminal fermentation, microbial growth, and populations of ruminal cellulolytic bacteria.Treatments consisted of two NFC types (starch and inulin) supplemented with 0 g/d (low RDP) or 1.56 g/d (high RDP) sodium caseinate.No significant differences existed among dietary treatments in the apparent disappearance of dietary nutrients except for dietary N, which increased with increased dietary RDP (P<0.001).Compared with starch, inulin treatments reduced the molar proportion of acetate (P<0.001), the acetate:propionate ratio (P<0.001), and methane production (P=0.006), but increased the butyrate proportion (P<0.001).Increased dietary RDP led to increases in production of total volatile fatty acid (P,0.014) and methane (P=0.050), various measures of N (P≤0.046),and 16s rDNA copy numbers of Ruminococcus flavefaciens (P≤0.010).Non-fibre carbohydrate source did not affect daily microbial N flow regardless of dietary RDP, but ammonia N production was lower for inulin than for starch treatments under high RDP conditions (P<0.001).Compared with starch treatments,inulin depressed the copy numbers of Fibrobacter succinogenes in solid fraction (P=0.023) and R.flavefaciens in liquid (P=0.017) and solid fractions (P=0.007), but it increased the carboxymethylcellulase activity in solid fraction (P=0.045).Current results suggest that starch and inulin differ in ruminal volatile fatty acid fermentation but have similar effects on ruminal digestion and microbial synthesis in vitro, although inulin suppressed the growth of partial ruminal cellulolytic bacteria.