Embodiments of the invention relate generally to graphene fibers and, more particularly, to graphene fibers comprising intercalated large-sized graphene oxide (LGGO)/graphene sheets and small-sized graphene oxide (SMGO)/graphene sheets having high thermal and electrical conductivities and high mechanical strength. In one embodiment, the invention provides a graphene fiber comprising: a plurality of intercalated graphene sheets including: a plurality of large-sized graphene sheets; and a plurality of small-sized graphene sheets, wherein at least one of the plurality of small-sized graphene sheets is disposed between at least two of the plurality of large-sized graphene sheets.