Widely applied total body irradiation (TBI) for bone marrow transplantation and treatment of lymphoma and disseminated other malignancies, can be effectively performed when uniformity of dose distribution is established. Special considerations for TBI dosimetry are required as several problems associated with very large field size and long source-skin-distance (SSD) arose. For the TBI with LMR-13 linear accelerator, radiobiological aspects, techniques and methods were discussed and through direct phantom measurements, basic radiation physical data were obtained. Following results are presented. 1. Adequate field size up to 130$ imes$130cm for the whole body at 4.1m from the source can be provided. Though about 20% decrease of beam intensity in the corners of the field was detected, uniformity of dose distribution can be established with the patient seated with flexion of the legs. 2. Lateral opposing two portals were used as the treatment geometry. 3. The bed made of styrofoam was devised for the fixation of the patient position and thick hospital gown itself acted as the bolus. 4. Compensation filters were designed for the uniform dose distribution and the central axis dose difference less than $pm$5% was achieved. 5. For the further repeated measurements, humaniod phantom in sitting position, as large as a man, was made with the use of soft tissue equivalent paraffin compound and lung tissue equivalent cork. 6. Dosage at several anatomic locations were monitored using TLD chips.