Thick tempered martensitic steel plates were friction stir processed at different tool traverse and rotational speeds. Friction stir processing led to formation of finer martensite phase, ferrite, and higher angle grain boundaries, which resulted in higher hardness of the processed samples. However, the macro mechanical results indicated that the tensile strength and elongation were reduced after friction stir processing, was due to the formation of ferritic bands and WC debris in the processed zones. The brittle fractured surfaces of processed samples confirmed the negative effect of friction stir processing on tensile behavior, which contained more cleavage than dimples compared to base material. The outcome of this study can be used for surface modification of thick martensitic materials for industrial applications.