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Stress relaxation, the decreasing trend of stress at a given strain along with time, is an essential mechanical behavior for any structural material. Graphene fiber (GF), a new kind of carbonaceous fiber, has emerged prominent mechanical performance and multifunctionality since first invented in 2011. However, most of efforts were focused on the improvement of mechanical strength and electrical/thermal conductivity and dynamic mechanical behavior has been ignored. Here, we studied the stress relaxation behavior of GF and revealed a severe stress drop for nascent GF. We found that the drop in stress during relaxation decreased with the promotion of the orientation degree and crystallinity. Plasticization stretching strategy and graphitization process were presented to relieve the relaxation of GF, accompanying with excellent mechanical strength. The optimized GF exhibits best stress relaxation property with 99% stress remained after relaxation. This work reveals an essential relaxation behavior of GFs and develops effective processes to conquer stress relaxation, making GFs for practical applications as structural materials. [Display omitted] [ABSTRACT FROM AUTHOR]