Background: Although endometrial cancer is not ranked among the ten most common cancer types in Iran, it is the 12th most prevalent in women and the third most common cancer of the female genital tract after breast and ovarian cancer. The mortality rate of endometrial cancer in Iran is 0.6 in 100,000 persons. Several studies have evaluated the correlation of preoperative CA125 in endometrial cancer with several surgicopathologic and prognostic variables, disease recurrence, and the need for lymphadenectomy. Recent data have suggested adjuvant extrafascial hysterectomy after neoadjuvant therapy instead of the initial radical hysterectomy for locally advanced disease. Therefore, it would be helpful to use a preoperative assessment, including serum CA125 measurement to predict the extent of the disease and plan a less complicated therapy. Objectives: To evaluate the cutoff value of CA125 in parametrial and cervical stromal invasion of endometrioid endometrial cancer. Methods: A sample of 128 endometrial cancer cases, surgically staged from 2012 to 2018 in Imam Hossein Hospital were evaluated. According to the exclusion criteria, 82 cases were finally analyzed. A receiver operating characteristic (ROC) curve was used to determine the cutoff value of preoperative CA125 for parametrial and cervical stromal involvement. Results: A high preoperative CA125 level was significantly associated with advanced disease stage, cervical stromal invasion, pelvic lymph node metastases, and higher grade (P < 0.05); the test showed a marginally significant correlation for parametrial invasion, which may be due to sample size limitation (P = 0.058). However, the correlation between CA125 and myometrial/lymphovascular invasions were not statistically significant (P = 0.112 and P = 0.168, respectively). The optimal cutoff value for preoperative CA125 was 45.5 u/mL in parametrial invasion (100% sensitivity, 89% specificity, 33.3% positive predictive value, and 100% negative predictive value), and 41.9 u/mL for cervical stromal invasion (87.5% sensitivity, 87.8% specificity, 43.75% positive predictive value, and 98.48% negative predictive value). Conclusions: Primary radical surgery may not be the first treatment approach in cervical/parametrial involvement. Neoadjuvant (chemo)radiotherapy and adjuvant extrafascial hysterectomy could be an alternative approach with fewer complications. Using preoperative CA125 along with physical examination and imaging modalities would be helpful in this regard. More investigations are needed to assess an agreed cutoff value for preoperative CA125 and endometrial cancer extension.