Viscosity of supercooled liquids and thermal properties in Zr_55+xCu_30−x−yNi_5+yAl₁₀ (x=0, 5, 10, y=0, 2, 5, 10) (at%) bulk metallic glasses (BMGs) have been examined. Viscosity has been measured by using a penetration viscometer at a high speed heating rate of 6.67 K/s in a high purity He-gas atmosphere. With the increase of the Zr-content from 55 to 60 at% and above, the viscosity of the supercooled liquids was drastically decreased from roughly η=10⁸ to 10⁷ Pa•s at 750 K. In this case, the glass transition temperature (T_g) was slightly decreased and the crystallization temperature (T_x) was largely increased due to the change of the main crystallization phase from metastable fcc-Zr₂Ni to stable bct-Zr₂Cu, resulting in a large increase of the ΔT_x (=T_x−T_g) value. Maximum ΔT_x values of 128 K and 172 K were observed in the Zr₆₅Cu₁₈Ni₇Al₁₀ BMG at heating rates of 0.67 and 6.67 K/s, respectively. It may be suggested that the drastic change of the viscosity and the main crystallization phase may be due to the structure change of the supercooled liquids in the compositional border between 55 and 60 at% Zr.