The objective of this experiment was to study the effects of dried spent mushroom Cordyceps militaris (SMCM) supplementation on digestibility, rumen fermentation, and blood metabolite parameters in goats. When the mushroom production and harvesting was complete, SMCM was collected fresh from a mushroom farm. Four 18-mo-old male crossbred (Thai Native × Anglo Nubian) goats with initial BW of 26.0 ± 1.40 kg were randomly assigned according to a 4 × 4 Latin square design to receive four different levels of SMCM at 0, 100, 200, and 300 g/d. There were quadratic effects (P < 0.06) on apparent digestibility of DM, OM, CP, NDF, and ADF among treatment groups, and greater values for the goats fed SMCM at 100 g/d were observed. A quadratic effect in energy intake (ME, Mcal/kg DM) (P = 0.02), with greater values for the goats fed SMCM at 100 g/d, was also observed. Rumen temperature was similar among groups (P = 0.23), whereas feeding of SMCM linearly decreased (P = 0.001) ruminal pH with increasing SMCM supplementation. Supplementing SMCM linearly decreased plasma concentration of cholesterol (P = 0.01), and there was also a tendency of reduction in plasma concentration of triglyceride (P = 0.10), with greater values for the goats fed SMCM at 100 g/d. There were quadratic effects on plasma of total protein, HgB, and MCHC among treatment groups. However, there were quadratic effects on fecal N (P = 0.04) and nitrogen retention (P = 0.07) based on g/d/animal or percentage of nitrogen retained (P = 0.01) among treatment groups, and greater values for the goats fed SMCM at 100 g/d were observed. Based on this experiment, it could be concluded that supplementing goat diets with SMCM at 100 to 200 g/d results in improved apparent digestibility of nutrients and blood metabolites, suggesting that SMCM has positive functions as a feed additive to improve energy substance metabolism and contribute to glyconeogenesis.