It will consist of 2 phases. An open-label preliminary phase lasts for 2 weeks, during which paroxetine is titrated up to 20mg/day (Day 5). At Week 2, patients will be evaluated by HAMD-17. The patients who have achieved early improvement (the decrease of HAMD-17 total score ≥ 20%) will be discontinued. The patients who have not achieved early improvement (the decrease of HAMD-17 total score ＜ 20%), will be randomized into three treatment arms [1.Mirtazapine (30mg/d); 2.Paroxetine (20mg/d); 3.Mirtazapine (30mg/d) +Paroxetine(20mg/d)]. In double-blind treatment phase (consist of 6 weeks), patients will be evaluated at Week 3, 4, 6 and 8.