The success of radioation therapy depends on exact treatment of the tumor with significant high dose for maximizing local control and excluding the normal tissues for minimizing unwanted complications. To achieve these goals, correct estimation of target volume in three dimension, exact dose distribution in tumor and normal critical structures and correction of tissue inhomogeneity are required. The effect of therapy oriented CT (planning CT) were compared with conventional simulation method in necessity of planning change, set dose, and proper distribution of tumor dose. Of 365 new patients examined, planning CT was performed in 104 patients (28%). Treatment planning was changed in 47% of head and neck tumor, 79% of intrathoracic tumor and 63% of abdmonial tumor. In breast cancer and musculoskeletal tumors, planning CT was recommended for selection of adequate energy and calculation of exact dose to critical structures such as kidney or spinal cord. The average difference of tumor doses between CT planning and conventional simulation was 10% in intrathoracic and intra-abdominal tumors but 20% in head and neck tumors which suggested that tumor dose may be overestimated in conventional simulation. Although some limitations and disadvantages including the cost and irradiation during CT are still criticizing, our study showed that CT planning is very helpful in radiotherapy planning.