Testis is an important organ in which spermatogenesis happens and nutrition and protection for the development of sperm is provided. In insect, there usually has two bilateral symmetry testes during larval stages, but in some insects of Lepidoptera such as agriculture pest Spodoptera litura, two testes fuse together during larval to pupal metamorphosis and one single testis is formed at pupal stage. However, in lepidopteron economic insect silkworm, two testes do not fuse during metamorphosis development and always keep separated in all developmental stages. In insect, fusion of testis in Lepidoptera may be a kind of reproductive mode formed after a long natural selection and evolution and may be benefit for the reproduction and enlargement of population. In this paper, testis types of Lepidoptera, the structure of testis, the process of spermatogenesis, fusion of testis, its biological significance and its potential application in pest control in agriculture are described and summarized.