In order to study the effects of two kinds of clinoptilolites (Calcic and Potassic) on ruminal parameters, four fistulated mature male sheep were used in a change over design as balanced latin square. The formulated diets(contain 60% concentrate and 40% forage) include: 1) control, 2)4% Calcic clinoptilolite, 3) control + 4% Potassic clinoptilolite and 4) control + 2%Calcic and 2% Potassic clinoptilolite. The experiment was comprised of four periods. Each period included adaptation (two weeks) and sampling (one week). Sampling was made from ruminal fluid for measurement of pH, ammonia nitrogen, total volatile fatty acids(tVFAs), ruminal cellulolytic and total bacterial population at 0, 3, 6, and 10hours after morning feeding. The results showed that Calcic clinoptilolite increased pH, tVFAs and ruminal cellulolytic bacterial population significantly (P<0.05), but its effect on total ruminal bacteria was not significant. Calcic clinoptilolite also decreased ruminal ammonia nitrogen at 3hours after feeding, and in general with using of calcic clinoptilolite its concentration in ruminal fluid had a lower fluctuation. Effects of potassic clinoptilolite on the mentioned parameters were not significant. In general the results indicated that using %4 calcic clinoptilolite changes ruminal parameters for increasing animal performance.