Purpose. To justify the use of the Lewis phenotypic stabi­lity factor (SF) to determine the stability of the manifestation of quantitative and economic value characteristics of national varieties of sweet potato (Ipomoea batatas L.). Methods. Field research on the comprehensive assessment of new sweet potato varieties was carried out in 2021–2022 at the Institute of Vegetable and Melon of the NAAS (Kharkiv Region). Phenological observations and biometric measurements of plants were carried out, as well as recording of meteorological data during the growing season, in particular during interphase periods of the culture. The stability of the studied traits was determined by finding the limit values of their manifestation (Xmax and Xmin) and further calculating the Levis phenotypic stability factor (SF). Results. The use of the Levis phenotypic stability factor (SF) made it possible to determine the stability of the manifestation of morphological features of sweet potato varieties (I. batatas), in particular, the quantitative characteristics of tubers: their number under the bush for the variety ‘Admiral’ is 5.3 pcs., for ‘Slobozhanskyi rubin’ – 3.7 pcs.; the average weight of the tuber is 254 and 283 g, respectively. The trait “sweet potato tuber shape index” ensured stability with a coefficient of 0.98 (‘Admiral’) and 1.03 (‘Slobozhanskyi rubin’). The stability of the manifestation of morphological features of sweet potato tubers was established for the varieties ‘Admiral’ and ‘Slobozhanskyi rubin’, which are common in Ukraine. They represent two maturity groups, and therefore differ in the duration of the tuber formation period. Thus, the early-ripening ‘Admiral’ has a vegetation period of 100–110 days; medium-ripening ‘Slobozhanskyi rubin’ – 110–120 days. Conclusions. The quantitative trait of sweet potato tubers of the ‘Admiral’ variety was more stable in terms of width; ‘Slobozhanskyi rubin’ variety – in terms length. Tuber length values varied within cultivars. The shape index of sweet potato tubers as an identification feature of varieties turned out to be variably unstable; the calculated Lewis phenotypic stability factor was lower than unity for both studied varieties.