Abstract: In the present study, variation in 20 phenotypic traits of 398 samples of Paeonia ostii T. Hong et J. X. Zhang from 15 seedling populations in six provinces were investigated and analyzed. Results showed that 10 traits, including branch number, ground diameter, plant height, stem length, crown length, crown width, flower branch, bud number, leaf length, and leaf width, were significantly affected by plant age. With the increase of plant age, the mean values of leaf length and width showed a decreasing trend, whereas the other eight traits showed an increasing trend. The average phenotypic differentiation coefficients (Vst) between populations were between 0 and 50.14%, with an average value of 27.62%. Annual growth length exhibited the largest variation, whereas number of carpels and annual diameter showed the smallest. Principal component analysis was conducted for the 10 traits affected by plant age, including length of one-year-old branches, bud height, diameter of one-year-old branches, length of terminal leaflets, diameter of two-year-old branches, width of terminal leaflets, leaf number, carpel number, length of two-year-old branches, and petiole length. Results showed that the first six characters were the main traits accounting for the phenotypic differences. There were some correlations between phenotypic traits, with the 10 traits affected by plant age being divided into four groups by cluster analysis. There was great variation among populations and within populations in these provinces, which provides a wealth of material for variety breeding and germplasm innovation for P. ostii.