This study was conducted to improve the microbial, chemical, and sensory quality of local butter offered in Qazvin City, Iran, with the Carum capticum essential oil (EO). Firstly, EO extraction was done by distillation method; its compounds were analyzed using GC-Mass. After determining the EO antimicrobial activity by disk diffusion and microdilution method, concentrations of 2 and 4% were selected for butter treatment. The samples were evaluated in terms of microbial (total bacterial count, Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, and yeast-mold), chemical (pH value and acidity), and sensory (color, flavor, odor, and overall acceptability), at days 1, 3, 5, 7, and 10. According to the results, thymol, p-cymene, and γ-terpinene were the main components of the EO. The minimum inhibitory concentration (MIC) ranged from 0.05 to 0.125 %; the lowest MIC was related to E. coli and the highest was related to Candida albicans. Microbial evaluation of treated butter showed that the colony counts of examined organisms on days 3, 5, 7, and 10 decreased significantly (P<0.05) in the presence of 4% EO. The concentration of 2% EO significantly (P<0.05) reduced the organisms' population except on the third day. In addition, the EO reduced the acidity and increased the pH value in different storage days (P<0.05). The sensory evaluation also showed more acceptability of EO-containing butter (P<0.05). Considering the effects of C. Copticum EO in improving butter quality and its abundance as a native plant in Iran, it seems a proper candidate for replacing chemical preservatives and producing new products. پرونده مقاله