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Background and objectives: The insulin/insulin-like growth factor (IGF) system has many receptors, signaling pathways, and ligands, making it highly complex. IGF1 increases the function of germ cells by increasing the synthesis of spermatogonia DNA and slowing down apoptotic cells. One of the most important objectives of this study was to investigate the function of fennel on the expression of the IGF1 gene in Kermani sheep testes. Methods: The study involved three levels of fennel in the diets (20, 10, and zero g/kg dry matter), and tissue sampling was performed using testis tissue. Samples were rapidly placed in liquid nitrogen before being stored at -80°C. Then, total RNA was extracted and for the IGF1 gene (target) and GAPDH gene (reference), the real-time polymerase chain reaction was applied. Results: The outcomes displayed that increasing levels of fennel in the ration significantly (p < 0.05) increases the weight of the testis (0.35 kg and 0.36 kg at the levels of 1% and 2% fennel, respectively) compared to rations without fennel (0.29 kg at the levels of 0%). Association between testosterone and fennel feeding in studied lambs showed that adding fennel to the diet significantly (p < 0.05) increasesthe concentration of blood testosterone (3.5 ng/dL and 4.4 ng/dL at the levels of 1% and 2% fennel, respectively) compared with rations without fennel (1.7 ng/dL). The results show that adding fennel to the diet significantly (p < 0.05) increases the amount of IGF1 gene expression in the testis (2.5 and 2.7 at the levels of 1% and 2% fennel, respectively) compared with rations without fennel (1 at the levels of 0% fennel). Conclusions: Fennel has an affirmative effect on gene expression in the testis and can be added to sheep food rations to progress reproductive functions (by stimulating Leydig cell steroidogenesis, producing more sperm and testosterone, and growing and developing the testis). [ABSTRACT FROM AUTHOR]