Objective To study the clinical value of computed tomography (CT) and magnetic resonance imaging (MRI) in the delineation of target area of patients with nonsmall cell lung cancer (NSCLC) before radiotherapy. Methods The clinical data of 146 patients with NSCLC radiotherapy were retrospectively analyzed. They were divided into CT group (n=74) and MRI group (n=72) according to different imaging methods for target area delineation. The delineation volume [gross tumor volume (GTV), clinical target volume (CTV), planned target volume (PTV)] and lung irradiation dose [dose≥5 Gy volume fraction (V5), V20, mean lung DOSE (MLD)] were analyzed in the two groups. Results The levels of GTV, CTV, PTV, V5 and V20 in CT group were significantly higher than those in MRI group (P<0.05), but there was no significant difference in the MLD level between the two groups (P>0.05). Conclusion In patients with NSCLCbefore radiotherapy, MRI has smaller delineated target area than CT, and has more accurate guiding irradiation regimen, and it can reduce the risk of radiation damage in normal lung tissues, and it is beneficial to improve the efficacy and safety of radiotherapy.