This study was conducted to investigate the effects of grain (barley and wheat) size on rumen fermentation characteristics in fattening Dalaq breed lambs of the study was (3×2) completely randomized design with 6 treatments including; milled barley grain with sieve number two, milled barley grain with sieve number eight, unmilled barley grain, milled wheat grain with sieve number two, milled wheat grain with sieve number eight and unmilled wheat grain were plotted with 5 repetitions. Animals were housed in individual pens for 84 days. Rumen pH was not affected by experimental treatments. There was not significant (P>0.05) effect of grain paricle size on NH3-N ammonia nitrogen and microbial count although though, processing type it was affected by the type of processing and in the treatment of barley grain it was more than wheat. Also in the type of processing, sieve No. 2 produced more ammonia nitrogen. Counting of pro-tozoa was significant in treatments containing barley and wheat and in barley treatment it was more than wheat (P0.01). The total concentrations of volatile fatty acids (VFA) was not affected by the type of grain. Mi-crobial nitrogen and microbial protein produced in the rumen were significantly affected by experimental treatments and both of them were more than wheat in the treatment of barley seeds (P<0.01). Allantoin and uric acid were not affected, and absorbed purine and xanthine and hypoxanthine were affected by the grain effect. The activity of carboxy methyl cellulase enzyme was not affected by the treatments although micro-crystals cellulase enzyme in intracellular and total was affected by the treatments and it was more in barley. Manuscript profile