Aging causes thymus involution, and genes in thymus play an important role in the development of the immune system. In this study, we compared genes expressed in thymus of neonatal and peripubertal rats using annealing control primers (ACPs)-based GeneFishing polymerase chain reaction (PCR) and semiquantitative reverse transcription (RT)-PCR. We identified 10 differentially expressed genes (DEGs) with 20 ACPs. Of 10 DEGs, bystin-like, collagen type V alpha 1 (COL5A1), and T-cell receptor beta-chain segment 2 (TCRB2) that are related to immune-function were detected in rat thymus. Bystin-like and TCRB2 were up-regulated, while COL5A1 was down-regulated in peripubertal thymus. Semiquantitative RT-PCR confirmed postnatal changes in expression of bystin-like, COL5A1, and TCRB2. These results suggest that bystin-like, COL5A1, and TCRB2 could regulate immune function controlled in thymus as age increases.