Abstract： Objective To measure setup errors and calculate planning target volume(PTV)margin for patients with nasopharyngeal cancer(NPC)by using Elekta linear accelerator,and to investigate the effects of different PTV margins on radiotherapy dose. Methods The setup errors of the included NPC patients,who received radiotherapy,were statistically analyzed,and the margins were calculated by PTV margin formula. Then,the calculated PTV margins were imported into patient plan to compare the dosimetric difference with the conventional PTV margins. Results The setup error of NPC patients on the three?dimensional direction,X,Y and Z axis,was (-0.3 ± 1.5)mm,(1.7 ± 1.9)mm and (-1.7 ± 1.1)mm, respectively. The calculated PTV margin on the X,Y and Z axis was 3.6 mm,3.6 mm,3.1 mm,respectively. When the PTV margin was 3.6 mm,the D1cc[(5 080 ± 108.3)cGy]and D2%[(5 164 ± 93.5)cGy]of brainstem,D?max[(4 382±212.4)cGy]and D1cc[(4 010±72.2)cGy]of spinal cord,left crystal D?max [(532±32.2)cGy]and right crystal D?max[(550.9±37.2)cGy],left parotid gland V35[(51.63±3.2)%] and right parotid gland V35[(53.15±2.7)%]were found. When the PTV margin was 5mm,the D1cc&nbsp;[(5 240±103.1)cGy]and D2%[(5 380±95.2)cGy]of brainstem,D?max[(4 398±187.4)cGy]and D1cc [(4 070 ± 68.9)cGy]of spinal cord,left crystal D?max[(564.5 ± 33.3)cGy]and right crystal D?max [(554.6 ± 28.8)cGy],left parotid gland V35[(51.04 ± 3.8)%]and right parotid gland V35[(53.12 ± 3.2)%] were found. There were statistically significant differences in the radiotherapy dose of brainstem D1CC and D2%,spinal cord D?max and D1CC,right eye crystal D?max between the PTV margin of 3.6 mm and the conventional margin of 5 mm(P<0.05),whereas no statistically significant differences were found in the dose of left eye crystal D?max,parotid gland V35%,PGTVnd,PGTVnx and PTV2 (P>0.05). Conclusion Calculating the PTV margin for NPC patients in radiotherapy and applying it to clinical treatment favors the improvement of radiotherapy gain ratio.