Objective To study the set-up accuracy in radiotherapy of thoracic neoplasms by improving the body immobilization method. Methods Fifty patients with thoracic neoplasms were randomly divided into conventional group (without electrode paste) and improved group (with electrode paste). Using simulator for position calibration and center field digital image reconstruction from treatment planning system.Then compare the set-up accuracy of two groups with different body methods by grouped t-test. ResultsSet-up error in the left-right, superior-inferior, anterior-posterior direction were 2.5±1.5 and 2.4±1.4(P=0.010),4.4±2.0 and 2.2±1.2(P=0.000),2.2±1.3 and 2.1±1.0(P=0.100) in conventional group and improved group,respectively. Conclusions The improved body immobilization method improves setup accuracy in radiotherapy for thoracic neoplasms which also will be effective for clinical treatment.