In order to determine the mode of inheritance, genes action, general and specific combining abilities and effect of various environmental conditions on genetic parameters of harvest index and biological yield in bread wheat, diallel crosses design with eight parents was used. Parents and F1 progenies were sown in a randomized complete block design with three replications under drought stress and non-stress conditions in experimental field of Shahrekord Agricultural Research Center during 2011-2013 growing season. The data were analyzed according to method of Hallauer and Miranda as well as fixed model of Griffing’s method II. Jinks-Hayman model was used to estimate broad and narrow-sense heritabilities and mean degree of dominance. There were significant differences between genotypes for mentioned traits in both conditions. Studying mean square of general combining ability (GCA) and specific combining ability (SCA), the ratio of GCA to SCA mean square and portion of additive and dominance variances showed importance of both additive and non-additive genes effects for harvest index in both conditions, but in biological yield heredity, additive effect was more important. Estimating broad-sense and narrow-sense heritabilities showed low efficiency of harvest index and high efficiency of biological yield for selection programs in both conditions.