The extent of heterosis has been measured from the diallel cross over the mid and better parent in Indian mustard [Brassica juncea (L.) Czern & Coss]. Twenty- one F1 crosses along with seven diverse parental genotypes planted at the Division of Plant Breeding and Genetics, Sher-e-Kashmir University of Agricultural Sciences and Technology, Jammu, during the year 2013-2016 were evaluated for twelve characters, including days to bloom, days to reproductive phase, primary branches per plant, secondary branches per plant, pod intensity, seed yield per plant (g), harvest index (%), plant height (cm), pod length (cm), seeds per siliqua, seed weight per siliqua and 1000-seed weight (g). Analysis of variance revealed considerable genetic variability among parents and F1 crosses for all the traits except seed weight per siliqua. Heterosis ranged from 3.41(PM-21 x PM-24) to 101.34 per cent (PM-22 x Pusa-Karishma) for yield per plant. The present study also revealed that the crosses viz., PM-21 ×PM-22, PM-21 ×Pusa-Karishma, PM-22 ×PM-24, PM-22 × Pusa-Karishma and PM-24 × Pusa-Karishma had high postive heterosis for seed yield per plant.