Scientific drilling vessel Chikyu faced drill pipe failure during offshore commissioning of drilling control and instrumentation system upgrade work. The failure occurred with cyclic bending stress which was enough low to prevent fatigue failure according to S-N curve obtained from full-scale fatigue tests. Material tests and observation of a remaining part of the failed drill pipe, investigation of onboard data observed and measured during the operation, and fatigue analysis based on the material tests data and stress amplitude determined by both the onboard observation and the simulation of drill pipe motion are examined to investigate the cause of the failure. All three examinations provide that the number of cycles of bending stress to fail is in the order of 10⁴. The examinations also provide an evidence of cause of initiating cracks. This study presents the results of examinations and discusses the cause of drill pipe failure by verifying and complementing the results. It finally concludes that micro cracks were initiated owing to heat and stress, then cracks grew with cyclic stress, and it consequently reached to failure owing to the drill pipe motion including Magnus effect.