The rheological properties of carbon black gel ink were investigated by measurement the relationship between shear stress and shear rate. The fitting to the common rheological curve of gel ink with Hersegel-Bulkley equation was introduced. And the rheological parameters used to characterize the writing properties of gel ink were proposed. The results indicated that the gel ink system possessed strong shear-thinning ability. And the writing performance of the gel ink could be characterized with rheological parameters comprehensively.