Heterosis over better parent for seed yield and its component traits were studied in 28 crosses derived from a diallel mating involving 8 diverse parents of lentil. For seed yield heterosis ranged from −1.73 to 48.35 (%). Twenty two crosses had positive and significant heterosis for seed yield, out of them 9 crosses viz., DPL 62 × K 75, PL 4 × K 75, B 18 × Lens 830, PL 4 × B 18, B 18 × K 75, PL 4 × DPL 62, DPL 62 × L9–12, DPL 62 × B 18 and K 75 × Lens 830 were the best hybrids having high heterosis for seed yield/plant, plant height and pods/plant. It was revealed that high heterosis was attributed due to luxuriant plant growth coupled with high frequency of pods seed. Considering heterosis, inbreeding depression, sca effect of cross and gca effect of parents involve in crosses indicated that grossly non additive gene action played major role for expression of high heterosis for seed yield.