Graphene oxide, which is 2-dimensional material, has lots of oxygen functional groups making the graphene oxide be dispersed in water and organic solvents. And the dispersed graphene oxide shows liquid crystallinity. The liquid crystalline graphene oxide is possible to be spun in fiber and because the fiber has excellent tensile strength and electrical conductivity, graphene fiber is considered as a candidate material for smart fabric which is next-generation application. To be applied on smart fabric, graphene fiber needs to show excellent elongation performance. Thus, many researches have been conducted to improve mechanical performance of graphene fiber. In this work, nanodiamond, which has a spherical structure in nanoscale, was introduced to fabricate graphene/nanodiamond composite fiber. The composite fiber showed improved elongational performance maintaining excellent tensile strength and electrical conductivity. By atomic force microscopy and rheology measurement, it is confirmed that nanodiamond worked as nano-bearing between graphene sheets resulting in reduced frictional force. Through this work, it is suggested that improved graphene/nanodiamond composite fiber is applicable to smart fabric.