A deep-sea riser drilling vessel is now under construction in Japan for the purpose of scientific drilling and will be completed in 2005. The target water depth of the vessel is 2, 500m for the time being and will be extended to 4, 000m in future. In the field of offshore oil drilling, it is confirmed that the target water depth of 3, 000m with using steel riser pipe will be possible. However, the drilling up to the water depth of 4, 000m has not been experienced at all and will have a lot of technical issues. One of the most important issues is to make the weight of the riser pipe lighter and its axial rigidity higher. The purpose of this research is to make clear the design concept of the mechanical joint between the CFRP pipe and the steel connector and develop a CFRP riser pipe capable of drilling in 4, 000m water depth. Here, a prototype of CFRP riser pipe is designed and five experimental models are fabricated and tested. The experimental results are compared with those of FEM analysis and the findings are reported in this paper.