Objective To explore the changes of MRI images before and after intensity modulated radiotherapy for nasopharyngeal carcinoma and the application value in efficacy evaluation. Methods 50 patients with nasopharyngeal carcinoma who were admitted into the hospital during January 2012 to August 2014 were studied. All patients were treated with intensity modulated radiotherapy and received MRI examination before and after the treatment. The treatment effective rate was statistiacally analyzed. The slope of MRI curve was compared between fibrosis and recurrent patients after radiotherapy. Results Before the treatment, uniform signal was on T1WI, high signal on T2WI of and high signal on DWI images. After the treatment, uneven high signal was on T2WI, low signal on T1WI and signal decreasing on DWI sequence. The boundary was clearer than that before the treatment and the enhanced scan showed inhomogeneous enhancement. The effective rate of radiotherapy was 92.0%(46/50). The slope of the fibrosis group was significantly lower than that of the recurrence group and the difference was statistically significant (P ＜0.05). Conclusion The application of MRI in nasopharyngeal carcinoma can be used to accurately display the range of tumor and radiotherapy target region. According to the change of tumor morphology, the effect of intensity modulated radiotherapy is evaluated. Recurrence and fibrosis are differentiated. It is of significant value in the evaluation of clinical curative effect and prognosis of nasopharyngeal carcinoma, which is worthy of promotion.