Symbiotic nitrogen fixation in peanut offers an alternative way to reduce chemical nitrogen fertilization. The objectives of this study were to estimate general combining ability (GCA) and specific combining ability (SCA) effects for traits related to N2-fixation and agronomic traits and to identify the best parents for use in peanut breeding programs aimed at improving nitrogen-fixing ability. Full diallel (F2) crosses of peanut with reciprocals involving five parents were planted in a randomized complete block design with four replications. GCA mean squares were more important than SCA mean squares for plant dry weight, nodule dry weight, leaf color score, seed weight per plant, shelling percentage and harvest index. SCA mean squares were significant for pod weight per plant, seed weight per plant and 100 seed weight. SCA mean squares were more important than GCA mean squares for pod weight per plant, and equally important for seed weight per plant. Reciprocal mean squares were significantly important for 100 seed weight, shelling percentage and harvest index. KKU 72-1, (A3-1-1 (Bc 154) (PI 268770) and 42-G-105 (PI 269 109) were identified as good general combiners for traits related to N2-fixation, plant dry weight, nodule dry weight and leaf color score,whereas A3-1-1 (Bc 154) (PI 268 770) and KKU 72-1 were the best parents for 100 seed weight. A3-1-1 (Bc 154) (PI 268 770) was also the best general combiner for shelling percentage and harvest index.
