Soft tissue sarcomas are group of malignant tumours arising from extra skeletal mesenchymal tissue. Presenting a patient with swelling over the posterior aspect of left arm for 6 months, gradually increasing in size for four months and rapidly increasing in size for last 2 months and not associated with pain. Peripheral pulses felt. Ultrasound showed large solid cystic intramuscular lesion arising from triceps muscle and Trucut biopsy showed poorly differentiated malignancy. Magnetic resonance imaging (MRI) left arm impression was large lobulated solid cystic space occupying lesion in triceps muscle, possibility of neoplastic lesion. Here the patient underwent wide local excision and reconstruction procedure followed by Radiotherapy, chemotherapy and regular follow up.