Objectives: Eradication of H. pylori infection could reduce the occurence or recurrence of gastric cancer and peptic ulcer diseases. However, it was estimated that 15-20% of patients would fail from first line standard eradication therapy and need second line rescue therapy. About 15-30% of patient would fail from second line therapy. Bismuth quadruple therapy and triple therapy containing levofloxacin are currently the recommended rescue therapies in the second line and third line treatment. Recently, our pilot study showed that modified sequential therapy containing levofloxacin achieved high eradication rate (95%) in the second line therapy. The regimen includes a PPI plus amoxicillin for 5-7 days, followed by a PPI plus levofloxacin and metronidazole for another 5-7 days. However, whether this modified sequential therapy containing levofloxacin is more effective than bismuth quadruple therapy in the second line and third line treatment remains unknown. Besides, the impact of these regimens on the antibiotic resistance , microbiota of gut flora, and metabolic factors remains unknown.