The advantages of silkworms as functional foods are well known and various products are being developed. In general, silkworms sold in the market include silkworm powder (3 days of fifth instars) and SukJam (7 days or more of fifth instars), In other words, product classification is made according to the maturity of the fifth instar silkworms. In this study, we analyzed the gene expression changes in the fifth instar silkworms and attempted to validate the use of deregulated genes in maturity analysis. After rearing BaekokJam, transcriptome analysis was performed on days 1, 3, 5, and 8 days of the fifth instar, and differentially expressed genes showing differences at each period were selected. Of the 31,841 contigs analyzed, 4012 contigs were identified with a log2 fold change of two or more between 5 and 8 days of the fifth instar. RT-PCR was performed for 18 contigs, which showed increased or decreased expression, but in c127159, c97909, c96974, c119920, c42251, and c80216 showed clear differences. To identify SukJam, a combination of the contigs c127159 (180 bp), c97909 (143 bp), and c80216 (120 bp) was amplified. Taken together, these results suggest that the harvest time of silkworms can be determined using gene expression pattern analysis.