Abstract: Objective To investigate the mechanism of insulin resistance and estrogen in stimulating the development of endometrial cancer. Methods A total of 30 patients who were treated in Xuzhou Central Hospital and pathologically diagnosed with endometrial cancer from January 2014 to January 2018 were enrolled and set as an endometrial group. Meanwhile, another 30 patients with normal endometrium were set as a control group. Their levels of estradiol in serum and pathological specimens were detected. Both groups were compared for the levels of serum insulin and insulin resistance incidence. Furthermore, type I endometrial cancer patients with type 2 diabetes who underwent surgery in Xuzhou Medical Hospital from January 2009 to January 2018 were selected. According to their therapy, they were divided into two groups: a metformin group, a metformin combination group, and a non-metformin group. The effects of metformin on the development of endometrial cancer in patients were examined. Then, BrdU labeling method was used to detect the proliferation of Ishikawa and HEC-1-A cells after treatment with insulin or metformin. Real-time PCR was used to detect the expression of c-fos and c-myc mRNA in the cells after treatment with insulin or metformin. Results Compared with the control, endometrial cancer patients produced remarkable increases in the levels of estradiol in the serum and cancer tissues as well as serum insulin level and insulin resistance rate (P<0.05). Furthermore, after metformin treatment, endometrial cancer patients presented significantly decreases in the proportion of poorly differentiated adenocarcinoma, distant metastasis rate and tumor proliferation rate, compared with those without metformin treatment. Exposure to insulin resulted in stimulated proliferation of Ishikawa and HEC-1-A cells as well as increased expression of c-myc and c-fos mRNA, which were then significantly inhibited after combined treatment of insulin and metformin (P<0.05). Conclusions Insulin may play a synergic role with estrogen in some signaling pathways, so as to stimulate the development of endometrial cancer.