The present study was conducted to find out the dietary effects of mannan-oligosaccharides (MOS) supplemented diets on the production performance of four close-bred flocks (CBFs) of Japanese quail (Coturnix coturnix japonica) breeders. A total of 960 twelve-week-old birds of four CBFs were randomly divided into four groups (n = 240) with 12 replicates (n = 20). Birds were fed a corn-based basal diet or the same diet supplemented with 0.25%, 0.50%, and 1.0% MOS for 15 weeks. The authors analysed the data by two-way ANOVA techniques using SAS (Statistical Analysis System). Birds fed MOS-supplemented diets had significantly higher body and egg weight, egg mass, and egg number than the control group. Similarly, the feed conversion ratio (FCR)/dozen eggs, FCR/kg egg mass, and mortality were significantly lower in MOS supplemented groups. However, no significant effect of MOS supplementation was recorded on feed intake and egg production of birds. None of the parameters differed significantly among CBFs. The results showed that feeding MOS as a replacement for antibiotic growth factor may positively influence the production performance and health of Japanese quail breeders.