In this study, microstructure and mechanical properties of a friction stir welded 18Cr-2Mo ferritic stainless steel thick plate were investigated. The 5.4. mm thick plates with excellent properties were welded at a constant rotational speed and a changeable welding speed using a composite tool featuring a chosen volume fraction of cubic boron nitride (cBN) in a W-Re matrix. The high-quality welds were successfully produced with optimised welding parameters, and studied by means of optical microscopy (OM), scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD) and standard hardness and impact toughness testing. The results show that microstructure and mechanical properties of the joints are affected greatly, which is mainly related to the remarkably fine-grained microstructure of equiaxed ferrite that is observed in the friction stir welded joint. Meanwhile, the ratios of low-angle grain boundary in the stir zone regions significantly increase, and the texture turns strong. Compared with the base material, mechanical properties of the joint are maintained in a comparatively high level.