The complex surface impedance ( Z s =R s + i X s ) measurement was performed for almost optimally doped untwined YBa 2(Cu 1− x Zn x ) 3O y ( x=0.0088) to study the effect of Zn doping on the electronic state of the vortex core in the mixed state. The obtained value of vortex viscosity, η, and pinning frequency, ω p, are ∼1.9×10 −7 Ns/m 2, and ∼95 GHz, respectively, at 10 K. Comparing these with our previous result for the almost optimally doped untwined YBa 2Cu 3O y , we found that Zn impurity did not change the pinning constant κ p almost at all, whereas they decreased η. The former result suggests that pinning of vortices is dominated by defect other than Zn. On the other hand, the latter result is interpreted as the decrease in the life time of the quasiparticles in the vortex core.