Plants produce various secondary compounds mainly for protecting themselves from insects, fungi or bacteria. Plant secondary metabolites, namely caffeine (CA), is a powerful metabolic stimulant. Scarce experimentation has been conducted to evaluate the potential of CA to manipulate rumen fermentation. The objective of the current study was to determine the effects of using different doses of CA on fermentation profile and nutrient digestion when fed diets for lactating cows in continuous culture fermenters. We hypothesized that the addition of CA introduced at different doses into continuous cultures of mixed ruminal microorganisms will improve fermentation and nutrient utilization. Four treatments were randomly assigned to 8 continuous culture fermenters for 2 periods of 10 d. The experimental model administered according to a completely randomized block design. Treatments included a diet with a 40:60 forage to concentrate ratio with a control treatment (CON, no CA) and 3 treatments representing different doses of CA [CA-1 (50 ppm); CA-2 (100 ppm); CA-3 (150 ppm)]. Data were analyzed using the MIXED procedure of SAS as a randomized complete block design with blocks of period and fermenters; where linear, and quadratic contrasts where evaluated. Dry matter and OM apparent digestibility (AD) was decreased linearly as CA dose increase (P = 0.03). There was a quadratic tendency decrease on NDF and ADF AD as CA decreased (P = 0.06). Mean culture pH decreased linearly as CA increased in the diets (6.11, 6.00, 5.99 pH). The NH₃-N concentrations were high in CON and increased linearly with the addition of CA in the diets. The results of the current study suggest that adding CA up to 50 ppm can be beneficial for rumen fermentation and nutrient digestibility.