Phenotypic Stability Analysis for Seed Yield and its Associated Traits In advanced lines of Indian Mustard (Brassica juncea L. Czern and Coss)” was carried out to study the effects of different environments on seed yield and its associated traits, to estimate the magnitude of Genotype x Environment interaction and to find out the most stable and high yielding genotype among the selected lines of Indian mustard under different environments of different topography, soil texture and prevailing climatic conditions of Manipur. Fifteen genotypes were evaluated in 3 different locations (Andro, Iroisemba and Senapati) in RBD with 3 replications over two seasons viz; Rabi 2018-19 and 2019-20. The environment wise ANOVA revealed highly significant differences among all the genotypes studied for all the 11 characters of seed yield and its associated traits. The pooled ANOVA also indicated significant differences among the environments, genotypes as well as genotype by environment (GXE) interaction for all the traits. Genotype X Location interaction was observed as the main component for GXE interaction. Genotypes performed better in 2019-20 rabi season as compared to rabi 2018-19. Environment (E-5) i.e. Iroisemba, valley area was the best for expression of most of the characters studied. Stability Analysis using Eberhart and Russell indicated the significance of GXE (linear) for no. of siliqua/plant, no. of seeds/siliqua, siliqua length, no. of primary branches, no. of secondary branches, days to first flowering, days to 50% flowering, days to 80% maturity and 1000 seed weight except for plant height and seed yield per plot which shows the substantial amount of predictable G X E interaction for the expression of these characters in the selected genotypes. All the 15 genotypes were tested for 3 stability parameters, viz mean, bi and S2 di. The genotypes CAURMM-3, CAURMM-4, CAURM- 5, CAURM-4, PM- 25, CAURMM-1 and JM-1 were identified to be the high yielding and stable, hence they can be recommended for general cultivation under varied environments of Manipur. CAURM-1 CAURM-2, CAURM-3 had more stable characters although their yield was below the population mean, Therefore, proposed as promising genotypes for general cultivation under intensive input supply as they performed best in favourable environments, while NRCHB 101 found to be suitable for cultivation under poor environments.