Abstract: Objective :To diagnose occult micrometastasis of mediastinal lymph node in patientswith non-small cell lung cancer (NSCLC) and evaluate its clinical prognostic significance. Methods :Two hundred and forty-wo stations of mediastinal lymph nodes from fifty-eight patients with NSCLCwithout invasion of the tumor confirmed by routine histopathological examination (pNo) were studied to detect mRNA for MUC1 gene and determined occult micrometastasis, by using assays of reverse tran-scriptase-polymerase chain reaction (RT-PCR). The all patients were followed-up for three years. Sur-vival rate was calculated by method of aplan-Meier and survival difference between groups with andwithout occult micrometastasis group was compared with Log-Rank test. Results : The mRNA for MUC1gene was identified in twenty-hree stations of lymph nodes from sixteen patients, and occult mi-crometastasis was diagnosed in the patients (27.6%). TNM staging for those patients was up-gulatedfrom stage I_A-II_B to stage Ⅲ_A.Seven patients with occult micrometastasis were alive at three year(43.7%), thirty-me patients without occult micrometastasis were alive at three year (73.8%). Three-year survival rate was lower in the former than that in the latter (P<0.05). Conclusions : Occult mi-crometastasis in mediasinal lymph node of patients with NSCLC could be diagnosed by identification ofMUC1 mRNA. Occult micrometastasis of mediasinal lymph node in patients with pNo disease was anindicator of poorer prognosis.