Purpose: The aim of this study was to reduce the exposed dose of radiotherapy treatment planning computed tomography (CT) by using low tube voltage technique. Materials and Methods: We used tube voltages of 80 kV, 100 kV, and 120 kV, respectively. First, we evaluated exposure dose with CT dose index (CTDI) for each voltage. Second, we compared image quality indexes such as modulation transfer function (MTF), noise power spectrum (NPS), and contrast to noise ratio (CNR) of phantom images with each voltage. Third, CT to electron density tables were measured in three voltages and monitor unit value was calculated along with clinical cases. Finally, CT surface exposed dose of chest skin was measured by thermoluminescent dosimeter (TLD). Results: In image evaluation MTF and NPS were approximately equal; CNR slightly decreased, 2.0% for 100 kV. We performed check radiation dose accuracy for each tube voltage with each model phantom. As a result, the difference of MU value was not accepted. Finally, compared with 120 kV, CTDI_vol and TLD value showed markedly decreased radiation dose, 60% for 80 kV and 30% for 100 kV. Conclusion: Using a technique with low tube voltages, especially 100 kV, is useful in radiotherapy treatment planning to obtain 20% dose reduction without compromising 120 kV image quality.