Abstract: The effects of chemical treated corn silage (using urea and formaldehyde) on rumen ecosystem (pH, sedimentation and floatation, methylene blue reduction, total VFA, rumen ammonia) and blood metabolites (urea and glucose) in sheep were investigated. Sixteen sheep (41.85±4.6 kg) were used in a 4-treatment completely randomly design. The treatment contented: Corn Silage (CS), Corn Silage treated Urea(10 g kg-1)(CSU), Corn Silage treated Formaldehyde(4 g kg-1) (CSF), Corn silage treated urea (10 g kg-1) and formaldehyde (4 g kg-1) (CSFU). The ruminal pH had significant differences between treatments (p<0.05). The sedimentation andfloatation time, methylene blue reduction, total VFA showed no significant differences between treatments. The ruminal ammonia nitrogen in CUS was significantly more than the other treatments. The blood urea nitrogen in CSU and CSFU was more than the other treatments (p<0.05). There was no significant differences for blood glucose concentration among treatments. This study indicated that formaldehyde decreased ruminal ammonia nitrogen, resulting high escaped protein and offer users flexibility in formulating rations according to the productive performance of target animals.