Background: Malignant peripheral nerve sheath tumour (MPNST) is rare. It is a type of peripheral nerve sheath tumour that is cancerous (malignant). Most peripheral nerve sheath tumours are not cancerous (benign). A tumour is a lump or growth in the body. Objective: This paper reports a case of a malignant peripheral nerve sheath tumour revealed from the Magnetic Resonance Imaging (MRI) examination and other processes, with remarkable unusual size of the tumour. Case report: A 36 years old male patient with a three months history of massive right-thigh medial side swelling came to the clinic. The patient reported that he fell while walking before the beginning of the symptoms. He then visited the clinic with medial side thigh pain, limping, and massive swelling in the medial side of his right thigh. The provider recommended a triple assessment; 1- clinical, 2- laboratory, and 3- radiological. The clinical evaluation showed that the swelling was about 20 cm x 10 cm diffuse fusiform swelling, with tense stretched skin over cystic to a firm consistency. Conclusion: There is usually a poor MPNST prognosis, so the doctors must be sure about the diagnosis before treating the condition. Typically, there are three ways to treat MPNST: surgery, chemotherapy and radiotherapy.