This paper focuses on the latest research of diffusion-weighted magnetic resonance imaging (DWI) and the diagnostic benefit and prospects of DWI for thoracic tumors. DWI is reported to be useful for the differential diagnosis of malignancy or benignity for neoplasia in various organs. Diagnostic efficacy by DWI for pulmonary nodules and masses, the evaluation of N factors and M factors in lung cancer, is equivalent to or more than that of PET-CT. The diagnostic capability of whole-body DWI for the staging of clinically operable lung cancers is equivalent to that of PET-CT, and whole-body DWI is now available in clinics. Diagnosis by DWI for mediastinal tumors or lesions is equivalent to that of PET-CT. DWI is useful for the assessment of the therapeutic effect of chemotherapy and /or radiotherapy for lung cancer. The diagnostic performance of DWI for thoracic diseases is equivalent to that of PET-CT. A prospective randomized controlled trial for the comparison of diagnostic efficacy between PET-CT and DWI for lung cancer is necessary for accuracy.