Silkworms are agricultural insects producing silk fibres. Because of economic importance of sericulture, breeding programs are performed in order to improve bio-economic traits of lines (production and resistance traits) and also developing new lines containing interesting characteristics. Silkworms in Iran are reared twice a year, in spring and autumn. Spring is the natural rearing period of silkworms because of good environmental conditions. In autumn the quantity and quality of the cocoon produced by silkworm lines decreases due to increasing mortality and access to mulberry leaves with low nutritive quality. This decrease in performance will be lower in the lines which are genetically resistant to severe environmental conditions. Therefore, it is necessary to breed and develop new silkworm lines suitable for such a situation. It is obvious that silkworm lines having lower performance but higher resistance can be introduced for autumn season rearing. Appropriate breeding programs have to be defined and conducted for the autumn rearing lines. Thus, we have to estimate genetic parameters of autumn and spring lines separately when reared in each season. The present study was undertaken to identify heritability of important traits in the spring and autumn seasons for 6 silkworm lines.