Giant cell tumour (GCT) of flat bones of pelvis is extremely rare. GCT of the ilium and ischium represent less than 0.05% of all GCT. Iliac bone GCT has been treated traditionally by intra-lesion curettage with bone grafting, wide resection with or without reconstruction and hemi-pelvectomy in very aggressive tumours. None of the above treatments were without morbidities. Reconstruction using bone grafts and bone cement has also been inadequate. In GCT, where life expectancy is not decreased significantly, surgical treatment should be aimed at giving optimum functional outcome. We are reporting here a rare case of giant cell tumour of ilium bone in a 25-year-old female and its unique treatment approach. We designed a computed tomography (CT) based customized iliac prosthesis using Materialise Mimics and 3-Matic software. 3D model of pelvis was generated from the CT. After deciding the extent of resection on affected side, we virtually mirrored an identical portion of opposite ilium to the affected side. Connecting plates were made over the mirrored part and merged with it. Multiple relevant holes were made to attach various muscles to the prosthesis. Prosthesis was made in medical grade titanium by using Computerized Numerical Control (CNC) machine. The method is called as computer based subtractive manufacturing. Wide resection was done and the prosthesis was placed using multiple 3.5 millimetres screws through the connecting plates. Muscles were stitched to relevant holes using ethibond suture. Post-operative course was unremarkable. Patient was made to walk with full weight bearing after 5 weeks. Powers of abductors at 6 months is 4/5 and patient walks normally without a limp.