The article presents the results of phenological observations and morphometric measurements during the initial introduction of Paeonia tenuifolia L. under the conditions of the Republic of Mordovia. By the rhythm of flowering, the peony is thin-leaved - a spring-flowering plant. Flowering begins in mid-May and lasts an average of only 6 days. The longest is the fruiting phase (30-42 days). The timing of the onset and passage of individual phases depends on the weather during the growing season. The height of the plants, depending on the conditions of the year, ranged from 35 to 48 cm (43±2 cm), and the diameter of the flowers was 55-90 mm (72±5 mm) with the number of petals from 7 to 11. Under the conditions of Mordovia, over 11 years of observation, P. tenuifolia plants were fruited with the formation of seeds only 8 times (73%), due to the lack of pollination of flowers during inclement weather. The number of seeds in the fruits was small - 3-5 pcs./fruit. When sown, they had good germination (88-90%). Peony thin-leaved in the republic is unpretentious and promising for mass introduction using in landscape design.