We experienced a rare case of primary leiomyosarcoma of sigmoid mesentery. A 45-year-old woman was presented to us with left iliac fossa mass and discomfort for 4-month duration. CT scan of abdomen and pelvis revealed a huge mass 14 cm×14 cm×16 cm occupying left iliac fossa mimicked having a large left ovarian carcinoma. She was subsequently planned for elective total abdominal hysterectomy and bilateral salpingo-oophorectomy by gynaecology team. During laparotomy, a huge mass was revealed arising from sigmoid mesentery invaded to the left lower ureter. Curative resection was done and pathological findings show the tumour being leiomyosarcoma with immunohistochemistry tests on caldesmon, desmin, smooth muscle actin and CD34 reagent all positive. Clinicopathological and literature review of this rare primary leiomyosarcoma of mesocolon was discussed in our case presentation.