[Objectives]  The Fox transcription factor family has been shown to play a critical role in the development of insects. In this study, we investigate the function of Bombyx mori FoxG (BmFoxG) in the development of the testis, which may facilitate developing a strategy to increase the production of silkworms, or the control of pests by targeting their reproductive system. [Methods]  The full-length cDNAs of BmFoxG-1 and BmFoxG-2 ORFs were cloned from testis RNA samples by PCR and bioinformatics tools used to analyze the characteristics of the two deduced amino acid sequences. The expression levels of BmFoxG-1 and BmFoxG-2 in the testis of different developmental stages was determined by real-time quantitative PCR (QRT-PCR). BmFoxG-1 was then overexpressed in B. mori Bm12 cells and the potential genes that might be regulated by BmFoxG-1 detected by QRT-PCR. [Results]  The full length ORFs of BmFoxG-1 and BmFoxG-2 were 933 bp and 702 bp, respectively, and both proteins contain the conserved Forkhead domain with BmFoxG-1 having a longer C-terminal region. The mRNA expression level of BmFoxG-2 remained low in the testis of different developmental stages, but the transcript level of BmFoxG-1 changed during development and was always significantly higher than that of BmFoxG-2, suggesting that BmFoxG-1 may play a major role in testis development. Several cell cycle related genes, including BmVasa and BmCyclinA, were up-regulated in Bm12 cells overexpressing BmFoxG-1. [Conclusion]  BmFoxG-1 may participate in testis development in B. mori by regulating the function, or cell cycle processes, of germline cells.