Objective To compare the difference between the target volume delineated by CT and that delineated by positron emission tomography and computer tomography(PET-CT)in locally advanced pancreatic carcinoma (LAPC), and observe the impact of PET-CT on planning of three-dimensional conformal radiotherapy (3D- CRT). Methods Thirteen patients with locally advanced pancreatic carcinoma referred for radical radiation therapy, underwent PET-CT scanning. GTV was delineated on the CT imaging and PET-CT fused imaging respectively. The two images were used to develop the radiotherapy planning.Dose-volume histograms (DVH) parameters were selected to be paralleled to compare the quality of the two plans and the impact of PET-CT on radiation plan (RP) was further analyzed. Results (1) PET-CT image altered GTV and PTV: For all patients, the GTV_CT versus GTV_PET-CT was 57.3±22.9cm₃ versus 68.4±31.7cm₃,the CTV_CT versus CTV_PET-CT was 119.9±38.7cm₃ versus 132.7±47.5cm₃,the PTV_CT versus PTV_PET-CT was 288.6±52.1cm₃ versus 312.4±79.4cm₃.The difference was statistically significant. In 6 patients, changes were obvious. The average increase was 36.5%(22.15cm₃), due to the extension of the primary tumor beyond that defined by CT and additional lymph node metastases. (2) PET-CT image changed the parameters of DVH: to deliver the same target dose of 60 Gy in 30 fractions, all dose-volume characteristics altered in treatment planning, but there was no significant difference. Conclusions PET-CT image can reflect the relationship of the tumor and normal tissue more accurately. The application of PET-CT fused imaging on target volume delineation not only can provide a satisfactory the parameters of DVH, but also more precise on target volume delineation and planning of three-dimensional conformal radiotherapy for LAPC.