An elastic-viscoplastic constitutive mode was adopted to analyze asymptotically the tip field of a mode I quasi-statically propagating crack in rate-sensitive materials under plane stress condition. Under the assumption that the viscosity coefficient is a power law function of the rate of effective plastic strain, it was obtained through dimension match that the crack tip field possesses power law singularity. And the singularity exponent is uniquely determined by the power law exponent in the supposed viscosity coefficient. The elasticity, plasticity and viscosity of material at crack-tip only can be matched reasonably under linear-hardening condition. Variations of crack tip field characters according to each material parameter were discussed by means of numerical computation. The stress intensity is dominated by the material viscosity whereas the hardening coefficient has less significant influence on tip field. Furthermore, the solution can be transformed to the elastic-nonlinear-viscous one of Hui and Riedel if the limit case of zero hardening coefficient is considered.