This study aim to identify the therapeutic effect of azathioprine during infliximab(IFX) treatment in Crohn's disease (CD).Half of patients will receive azathioprine and infliximab combination therapy at the same time. The other half patients will be given infliximab therapy first and then receive azathioprine 14 weeks later.

Anti-tumor necrosis factor agents were widely used for many years and were proved to be most reliable drugs in Crohn's disease (CD). About one-third of the patients would develop a secondary lost of response after initiating IFX therapy in one year follow-up. Immunosuppressor was thought to be a useful agent to reduce loss of response rate of infliximab.But It remain unclear when should us add immunosuppressor during infliximab therapy.This study aim to find the most valuable timepoint to initiate combination therapy to maximize therapeutic effect and minimize the adverse effect.

Inclusion Criteria:

- CD patients need infliximab and azathioprine combination therapy

Exclusion Criteria:

- patients had lost response to infliximab or azathioprine previously patients intolerable to infliximab or azathioprine patients need to use 5-Aminosalicylates(5-ASA) patients had participated to other clinical study in two months