The purpose of this study was to investigate the effect of feeding Iranian propolis extract on perform-ance of Holstein dairy cows. In this study 18 dairy postpartum Holstein cows (120 days in milk (DIM) and body weight of 577±37.54 kg) were used. The experiment was carried out in a period of 29 d (10 d adaptation and 19 d sample collection). The data was analyzed considering a completely randomized design by the GLM procedure of SAS 9.1. In this experiment 3 treatments [A: TMR diet, B: TMR diet + Iranian propolis extract (IPE) 50% (67.50 g/day/animal) and C: TMR diet + IPE 75% (67.50 g/day/animal)] were used. The results of this study showed that adding different concentrations of IPE to dairy cow ration did not have any significant effect (P>0.05) on ruminal pH, but significant de-crease (P0.05) change some the blood parameters while statistically changed (P<0.05) the blood concentrations of aspartate ami-notransferase, albumin, glucose and protein. The phenolic compounds in propolis changed rumen ni-trogen ammonia, rumen short chain fatty acids, rumen microbial population, some blood parameters and fatty acid composition in the milk. These changes improve fermentation process and consequently affect significantly fatty acid composition in milk. پرونده مقاله