The effect of liquid protein feed (corn steep liquor; CSL contain 42% CP, DM basis) and its comparison with conventional protein sources (i.e. soybean meal; SBM and cottonseed meal; CSM) were evaluated in fistulated sheep fed with high wheat straw level included in diet (400 g/kg, DM basis). The study was carried out on three ruminal fistulated sheep in 3 × 3 Latin square design with 21-d periods (the first 14-d for adaptation period and the last 7-d for sample collection). The treatments were; (1) CSL; (2) SBM and (3) CSM included in basal diet. Microbial protein synthesis, ruminal fermentation, nutrients digestibility, hematology and blood metabolites were evaluated in the current study. Results show that intake was not differed among treatments (P > 0.05). However, NDF digestibility was increased in SBM fed sheep (P = 0.02). Total short chain fatty acid production was 73.68, 82.55, and 65.93 mmol for CSL, SBM and CSM, respectively (P = 0.03). Purine derivate as well as microbial protein synthesis were increased in sheep fed SBM (P = 0.02). The hematology of sheep was similar among treatments. Among blood metabolites only blood urea nitrogen was tended to be lower in SBM diet. In conclusion, results show that CSL as liquid protein feed could include in sheep fed high wheat straw diet with no negative effect and further works need to improve its nitrogen efficiency in animal nutrition.