The dynamic viscosities of molten Zr55Ni5Al10Cu30 and Zr50Cu50 alloys were measured by using rotating cylinder method under non-vacuum condition. According to the lnη~1/T curves, discontinuous changes were found and the activation energy was calculated. The activation energy of molten Zr55Ni5Al10Cu30 alloy is obviously larger than that of molten Zr50Cu50 alloy. Amorphous sample of Zr55Ni5Al10Cu30 with 3mm diameter was prepared successfully under non-vacuum condition, but there are crystallization phases in Zr50Cu50 amorphous sample with 2mm diameter. The relationship between viscosities of molten Zr-Cu based alloys and their glass forming ability (GFA) was discussed, and viscosities of molten Zr-Cu based alloys play an important role on their GFA.