Influence of maternal and sire-generation interaction effects on day-old chick' weight was investigated in two commercial broiler sire and dam lines, and in Mazandaran native fowls. The data were analysed through Restricted Maximum Likelihood (REML) procedures, using ASREML software. Ten different animal models were implemented and compared with likelihood ratio test. All models consisted of direct additive genetic effect but differed in combinations of maternal additive genetic, maternal environmental, sire-generation interaction and covariance between direct and maternal additive genetic effects. In spite of differences in the estimated genetic parameters from different models, the pattern of variations among the three strains was similar. In all strains, fitting direct additive genetic effects without considering other random effects resulted in overestimation of direct heritability (ranging from 0.422 to 0.754). The reduction in these estimates was substantially due to maternal effects, especially direct maternal additive genetic, which accounted for from 15.9% to 62.7% of phenotypic variations, either with or without inclusion of sire-generation interaction. Also, improvement in correlation between direct and maternal additive genetic effects (from -0.100 to 0.12, -0.13 to -0.06, and -0.21 to -0.16, for sire line, dam line and native fowls, respectively), due to the fitting of sire-generation interaction, indicated that its negative estimate is due to the implementation of an inappropriate statistical model.