Viscous flow behaviour of supercooled liquids and mechanical properties in Zr55+XCu30-XNi5Al10 (X=0, 5 and 10 at. %) and Zr65Cu18Ni7Al10 bulk metallic glasses (BMGs) have been examined. Viscosity has been measured by using a penetration viscometer at a high-speed heating rate of 400 K/min. With increasing Zr-content, Tg tended to decrease and Tx tended to increase, resulting Tx (=Tx-Tg) increaseing up to about 170 K under a heating rate of 400 K/min by the DSC analysis. The viscosity lowered about one order of magnitude, e.g., minimum value of the viscosity decreased from 3~4x106 Pa-s for the Zr55Cu30Ni5Al10 and Zr60Cu25Ni5Al10 BMGs down to 5~6x105 Pa-s for the Zr65Cu20Ni5Al10 and Zr65Cu18Ni7Al10 BMGs under the same heating condition. Compression tests were also carried out at a slow strain rate of about 1 x 10-4 s-1 at room temperature. With increasing Zr-content, the apparent yield stress was decreased but the total strain at fracture was increased significantly, suggesting that there is a strong relationship between viscosity and the mechanical properties in these BMGs.