The objective of the present study was to estimate the level of inbreeding coefficient during inbreeding of the pedigree of random mating and selected populations of two distinct races of mulberry silkworm, Bombyx mori (Lepidoptera: Bombycidae), in the silkworm germplasm. The six generation data of the two races, namely multivoltine Pure Mysore and bivoltine NB₄D₂, were studied for inbreeding depression coefficient using the residual maximum likelihood method, utilizing two statistical models by analyzing six quantitative traits, namely, larval weight, cocoon weight, shell weight, shell ratio, pupation rate, and filament length. The results of the present experiment demonstrated that the inbreeding coefficient was significant in Model 1 for most of the economic traits in the random mating populations of both the races compared to those of selected populations. These results suggest that during stock maintenance, application of rigid selection for increased numbers of generations helps to retain original characteristics of the pure races while reducing the deleterious effects of inbreeding. The significance of inbreeding coefficient is discussed with reference to the inbreeding of silk moths in the silkworm germplasm.