Assessing the effects of monensin on economic factors of milk (percents of protein and fat), and quality of milk fat during early lactation. Thirteen multiparous Holstein cows were blocked into a control and a treatment group from 3 weeks prepartum to 45 days postpartum according the number of their calving. The groups received similar diets. The treatment group received monensin added to grain mix at 340 mg/head/day during prepartum weeks, 160-480 mg/head/day during first two weeks postpartum and 350 mg/head/day during days 15-35 postpartum. Milk samples were obtained on days 15, 25, 35 and 45 postpartum. Milk volume during a standard period of 305 days and during the first 3 months of lactation, percentages milk protein and milk fat and iodine value of milk fat were determined. Data were studied with the SPSS statistical software using analysis of variance for repeated measures and independent t test. No significant differences were observed between control and experiment groups in volumes of milk during the first 3 months of lactation and the standard production of the entire lactation period. The natural decrease in the percentage of milk protein and fat postpartum was observed in both groups. However, the pattern of decrease in milk protein was slower in the treatment group. There was no difference between the groups in milk protein at various stages of sampling. Milk fat increased in the treatment group from day 25 of lactation and on day 45, it was significantly higher in the treatment group than in the control cows. The iodine index of milk fat was nonsignificantly higher in the treatment group throughout of the experiment. The present study showed that administration of monensin during periparturient weeks has no adverse effect on milk volume, percentage of milk protein and fat and the quality of milk fat.