Purpose: To reduce setup errors in the treatment of head and neck cancer patients during radiotherapy by applying quality control circle techniques. Materials and Methods: The following factors contributed to setup errors: bodily stiffness or medical conditions preventing patient movement, patient anxiety leading to localization errors, the disappearance of localization markers, and discrepancies between the positioning and reference locations for unknown reasons. Based on the principles of the Three occurrence and verifying true causes, 4 groups consisting of a total of 10 improvement strategies groups were created. Results: Following the implementation of these strategies, the means and standard deviations of setup errors in the X, Y, and Z axes improved from 1.67 ± 0.93 mm to 1.49 ± 0.58 mm (p =0.141), from 1.43 ± 0.66 mm to 1.08 ± 0.45mm (p < 0.001), and from 4.05 ± 1.8 mm to 1.16 ± 0.55 mm (p < 0.001), respectively. Additionally, 3D vector directional setup errors decreased from 5.00 ± 1.67 mm to 2.54 ± 0.65 mm (p < 0.001), and the Y-axis (roll) rotation error decreased from 0.76 ± 0.37° to 0.51 ± 0.24° (p < 0.001). For 3D vector directional errors >3 mm, errors decreased from 90% to 48%; for vector directional errors >5 mm, errors decreased from 43% to 0%. Conclusion: Setup errors were substantially reduced by the adopted strategies.