The effects of different inclusion level of whole corn plant silage to Napier grass were observed in determining rumen fermentation and microbial population in goats. Fifteen male Boer cross goats around six months old of approximately 18.54±1.83 kg of b.wt., were used as experimental animals. The goats were assigned into five groups with three goats per treatment group. The five treatment groups consisted of different proportions of Napier Grass (G) and whole corn plant silage (CS)-G/CS, (T1) 100/0, (T2) 75/25, (T3) 50/50, (T4) 25/75 and (T5) 0/100, respectively. The mean concentrations of rumen NH 3 -N (mg dLG 1 ) were not significant differences among the treatments, although T4 and T5 were slightly increased compared to other treatments. The total VFA production in the rumen fluid of the goat was not significantly different among the treatments, however; highest molar proportion of propionic acid and lowest proportion of acetic acid was observed in goat fed with T5 diets. Although the total bacteria population of rumen content was not significantly different among the dietary treatments, the population of R. albus, R. flavefaciens and F. succinogen showed significantly (p<0.05) among the treatments. The lowest population of methanogen and protozoa were detected in the rumen of goats fed T5 diet compared with other treatments. Thus, the animals fed with T5 diet showed the highest proportion of propionic acid and the lowest number of methanogen and protozoa population in the rumen. cellulolytic bacteria population in the rumen. According to Lettat et al. (2013), corn silage contains higher amounts of starch, which make it an interesting means to reduce methanogenic archea production as compared with alfalfa silage. It is well known that starch fermentation in the rumen favors propionate production at the expense of acetate and decreases ruminal pH, which reduces hydrogen availability and inhibits the activity of rumen methanogens (Martin et al., 2010;Hook et al., 2011). Rumen protozoal numbers are also often decreased in ruminants fed high-starch diets, which also reduces the transfer of hydrogen from protozoa to methanogens (Morgavi et al., 2012). In the ruminant, the production of methane reduces the energy availability of the animals. It is estimated between 2-12% of the gross energy in the feed is lost as a methane production (Johnson and Johnson, 1995). Thus, dietary manipulation to improve efficiency of nutrient utilization as well as to decrease the impact of ruminant production on the environment is an important goal for ruminant nutritionists. According to Lettat et al. (2013), increasing the corn silage in the diet of dairy cow increased propionate concentration but decreased ruminal pH, methane production and concentration of acetate and butyrate compared to alfalfa silage. De Campos et al. (2002) described that in vitro rumen fermentation profiles of 50:50 sugar cane/corn silage diets were higher compared to those of other combination. However, these investigations are more focused on inclusion of ...