Background and objective As other tumors, unresectabe lung cancer can cause many psychological problems to the patients, such as depression and anxiety. The present paper aims to evaluate the status of depression before and after knowing the state of illness in patients with non-small cell lung cancer of stage Ⅲ. Methods 43 casesof newly diagnosed non-small cell lung cancer (NSCLC) with stage Ⅲ were enrolled in the study. All the patients were distributed into three groups and given different intervention, that was completely unknowing the state of illness (group A), partly knowing the state of illness (group B) and completely knowing the state of illness (group C). Before and after knowing the state of illness, the depression status was assessed with the Hamilton depression rating scale for depression(HAMD). Results The mean total score of HAMD was unchanged both in group A and C, while significantly reduced in group B. The scores of anxiety somatization, cognitive disorder, retardation and feeling of despair were all significant lower in the group B after the patients partly knowing the state of illness, while the scores of sleep disorder was obviously higher in group C after the patients completely knowing the state of illness. The hypochondriasis was much severer in the group A, and in the group C, the score of suicidal idea became significantly higher after the patient knowing the diagnosis.Conclusion Depression is very common in the NSCLC patients with stage Ⅲ. Partly knowing the state of illness can obviously ameliorate the symptoms of depression, while completely knowing or completely unknowing the state of illness have no effect on relieving the patients' depression.