In this study four Zel rams, fitted with ruminal cannulas and fed at maintenance level. They were arranged in a 4´4 latin square design to determine the effects of different levels of tallow on microbial protein synthesis and protozoal population changes in the rumen and to determine whether changes in microbial protein synthesis were related to protozoal changes. The diets (were 55% concentrate, 45% forage), contained either no added fat (control), 2, 4 or 6% tallow on a the basis of DM. Urinary excretion of purine derivatives was determined to estimate the microbial protein synthesis. Ruminal holotrich and total protozoa were counted in 10 times after morning feeding. Microbial protein synthesis and excretion of purine derivatives were not affected by treatments but tended to decrease as fat level increased in the diet. ruminal holotrich protozoa population was not affected by added fat in all times after morning feedings but after 5 hours after morning feeding, Holotrich population in all times tended to decrease by increasing fat level. Rumen total protozoa population showed no significant differences between treatments at 0, 0.5, 1 and 2 hour after feeding but after this times it decreased by increasing fat levels (p<0.05).