The hypothesis tested in this study is that gefitinib / pemetrexed as maintenance therapy in patients with advanced (stage IV) EGFR mutation negative nonsquamous NSCLC who respond to 4 cycles of pemetrexed / cisplatin or pemetrexed/carboplatin as first-line therapy will achieve longer PFS than pemetrexed alone. We assume a median PFS of 4 months for patients receiving pemetrexed alone and the hazard ratio of pemetrexed alone compared to gefitinib/pemetrexed would be 0.42. It also indicates that the median PFS in gefitinib/pemetrexed group will be approximately 9.52 months. This is a 2-arm study in a 1:1 randomisation. Assuming an uniform accrual of 12-month with an addition of 12-month follow-up period, the total evaluable number of patients will be 52 to achieve a power of 80% and one-sided significance level of 0.025 to detect such difference between gefitinib/pemetrexed and pemetrexed alone.