Background: 3DCRT (three-dimensional conformal radiotherapy) and IMRT (intensity-modulated radiotherapy) has provided us with tools to delineate the radiation dose distribution of tumor targets. However, the precision of radiation can be compromised by respiratory motion, which usually limits the geometric and dosimetric accuracy of radiotherapy. The purpose of this study is to evaluate the impact of respiratory motion on dose distributions of 3D-CRT and dynamic IMRT by simulating the respiratory motion, and provide suggestions to optimize treatment planning. Materials and Methods: American Sun Nuclear Mapcheck 2D-ARRAY was placed on a moving platform to simulate the respiratory motion. The dose distributions were measured with a Sun Nuclear Mapcheck 2D-ARRAY on the moving platform. The motion cycle was 3.5s, the amplitude was ±3mm, ±5mm, ±10mm, ±15mm. Dosimetric distribution between 3DCRT and IMRT plans were contrasted byγ-passing rate analysis. SPSS 13.0 software was used for data processing and analysis. Results: The respiratory motion could blur the target dose distribution of 3D-CRT and IMRT. The pass rate (3% 3mm) in 3DCRT was larger than that in IMRT. The Mapcheck software reflected that, the respiratory motion largely affected the marginal dose distribution of 3D-CRT, while affected the whole target volumes of IMRT. Conclusions: Respiratory motion has a greater impact on the dose distribution of IMRT than on 3D-CRT. As for tumors with large motion amplitude, it is advisable to use 3DCRT rather than IMRT techniques.