This is a multi-center phase II randomized controlled study to assess the efficacy of Gefitinib 500mg in patients with IIIB/IV staged non-small cell lung cancer(NSCLC) with stable disease after a month treatment of 250mg Gefitinib by DCR,also PFS and OS.The side effect is evaluated as well.

Inclusion Criteria:

1. Histologically or cytologically confirmed stage IIIB/IV lung cancer(exclude patients confirmed by sputum cytology)
2. received at least one chemotherapy regimen
3. with stable disease after a month treatment of gefitinib(CT scan)
4. the radiotherapy focus is not the measurable disease within 4 weeks.the patients received palliative therapy can be included.
5. Age >18 years, either sex
6. with a measurable disease(longest diameters >=10mm with Spiral computed tomography (CT)and >=20mm with conventional CT) at least according to RECIST Criteria
7. WHO performance status(PS)<= 2 ( Patients Whit PS=2 Should not get worsen within 2 weeks Before Included)
8. N>=1.5×109/L, Plt>=1.5×109/L,Hb>=10g/dL
9. ALP<2.5×ULN.If ALP>=2.5ULN, AST&ALT should <1.5ULN(without liver metastasis) or <5ULN(with liver metastasis).TBIL<=1.5ULN，AST&ALT<2.5ULN(without liver metastasis) or <5ULN (with liver metastasis).
10. Signed and dated informed consent before the start of specific protocol procedures.
11. recruiting and receiving treatment in 5 days after last CT scan
12. Patients able to taken oral drug

Exclusion Criteria:

1. Life expectancy <= 12 weeks.Patients with metastatic brain tumors without symptoms or had been treated can be included.
2. experience of Anti-EGFR(the epidermal growth factor receptor) Monoclonal Antibody or small molecular compounds therapy such as erlotinib or Cetuximab.
3. can not take drug orally, have active peptic ulcer,half upper gastrointestinal or have dyspepsia.
4. allergic to gefitinib
5. Prior exposure to drugs without approval from this research or other study drugs within 21days before the 1st day taken Gefitinib 500mg or 250mg.
6. Pregnant or breast-feeding women
7. Severe systemic disease out of control such as unstable or uncompensated respiratory,cardiac,liver,renal diseases.
8. Previous or concurrent cancer that is distinct in primary site or histology from the cancer being evaluated in this study except of cervical carcinoma in situ,basal cell carcinoma within 5 years prior to study entry.