Estimates of genetic parameters of oil, protein and total glucosinolate contents, and seed size and weight per plant were determined using a linear mixed model analysis from 81 incomplete partial diallele crosses of 14 parental lines of Ethiopian mustard (Brassica carinata A. Braun) tested at two locations in Ethiopia. All the additive, dominance, and cytoplasmic components of variations and their interactions with the locations were important for all the traits. The additive and cytoplasmic main effects seemed to operate in opposite directions. The former accounted for more than 50% of the variation in oil, but only 13% in protein contents. The two traits, likewise, were genetically antithetical in that lines with increasing effects on oil had decreasing effects on protein or vice versa. This was further illustrated by the negative correlations observed between all the genetic components of the two traits. Notwithstanding this, however, lines with positive genetic effects on both traits were identified thus promising the prospect of improving both traits simultaneously. Oil content and seed size exhibited similar genetic characteristics and were positively correlated, implying the possibility of selecting high-oil genotypes based on their seed size.