Objective To investigate the effect of Janus kinase (JAK) inhibition by baricitinib on erosion healing in rheumatoid arthritis (RA) patients with active disease using high-resolution peripheral quantitative computer tomography(HR-pQCT). Hypothesis JAK inhibitor can lead to healing of existing erosion in RA patients with active disease. Design and subjects This is a 24-week, randomized, placebo-controlled, double-blind study. We plan to enroll 60 adult patients with active RA (Disease activity score 28-C-reactive protein \[DAS28-CRP\]\>3.2) and 1 bone erosion on HR-pQCT. They will be randomized 1:1 to receive JAK inhibitor (baricitinib 4mg once daily) or placebo for 24 weeks. Medications will be adjusted according to a standard protocol aiming to achieve low disease activity. Patients requiring biologic or other targeted synthetic disease-modifying-anti-rheumatic-drugs will be excluded. Study instruments HR-pQCT of the 2-4 metacarpophalangeal(MCP) will be done at baseline and 24 weeks. Inflammatory cytokine profile and bone cartilage interface biomarkers will also be checked at baseline and 24 weeks. Clinical response will be monitored using DAS28-CRP. Main outcome measures and analysis The primary outcome is the proportion of patients with erosion volume regression on HR-pQCT comparing the two groups by chisquare test.