Additional Study Information: This is a 6-week randomized, double-blind trial of 4 different non-steroidal anti-inflammatory drugs in patients with axial spondyloarthritis to compare the change of pain score from baseline at 4 weeks to the change of pain score from baseline at 6 weeks. Patients with ankylosing spondylitis or axial spondyloarthritis who fulfills the inclusion and exclusion criteria will be randomized into one of the four arms after an initial one week washout period, including: 1) indomethacin 75mg Q12H; 2) diclofenac 75mg Q12H; 3) meloxicam 7.5mg Q12H; 4) celecoxib 200mg Q12H. The treatment length will be 6 weeks. Primary outcome is the change of pain score from baseline to week 4 and to week 6.