[en] We have prepared Na-doped YBa₂Cu₃O_y (YBa_1.9Na_0.1Cu₃O_y+40 mol% Y211) (YBNCO) and Na-free YBa₂Cu₃O_y (YBCO) samples by the melt-textured growth method to study the effect of doped Na ion on flux pinning. The ac susceptibility curves as well as the hysteresis loops were measured for the samples. Then the effective pinning energy (U(T,H_dc,J)), irreversibility line (H_irr(T)) and critical current density (j_c(H_dc)) were determined, where T, H_dc and J are temperature, dc magnetic field and current density, respectively. We found that, with Na doping, the H_irr(T) line shifted to lower temperatures while the J_c(H_dc) and U(T,H_dc,J) became smaller. Hence Na ions play a negative role in the flux pinning of YBCO. The appearance of the second peak in the J_c(H_dc) curves and the enhancement of anisotropy in YBNCO further support this finding