The aim of the investigation was to determine the effect of a plant extracts mixture (Herbiplant CS) used as a feed supplement on fattener’s performance, carcass value and back fat quality. The preparation was made at the Research-Promoting Centre LNB Poland Ltd. in Kiszkowo and it was standardized with respect to the bioactive substances concentration. Herbiplant CS phytopreparation was applied in the form of loose powder to feed mixtures of a starter, grower and finisher type in the amount of 125 and 500 mg/kg (3 groups of fatteners). The animals were kept individually and fattened from 20 to 100 kg of body weight. An average daily live weight gain in control group within the whole period of fattening reached 896 g, and in the groups supplemented with Herbiplant CS was higher of 3.8 and 5.0%. An average daily feed intake by pigs in the subsequent stages of fattening was similar in all groups. Feed consumption per 1 kg of weight gain in the whole fattening period was 2.67 kg in control group, while in those supplemented with Herbiplant CS it was lower by 4.9 and 6.4%. An introduction of phytopreparation dose to feed mixtures had no effect on carcass meatiness and only the increase in loin “eye” was observed. The assessed phytopreparation did not significantly influence fatty acids profile. The contents of saturated (SFA), monounsaturated (MUFA) and polyunsaturated (PUFA) fatty acids were similar. Phytopreparation Herbiplant CS at a dose of 500 mg per 1 kg of mixtures improved performance indicators more considerably in the first period of fattening, while the other periods featured similar efficiency compared Herbiplant CS doses (125 and 500 mg/kg).