The influences of cooling rate on the phase constitution, microstructural length scale, and microhardness of directionally solidified Galvalume (Zn-55Al-1.6Si) alloy were investigated by directional solidification experiments at different withdrawal speeds (5, 10, 20, 50, 100, 200, and 400 µm·s^-1). The results show that the microstructure of directionally solidified Galvalume alloys is composed of primary Al dendrites, Si-rich phase and (Zn-Al-Si) ternary eutectics at the withdrawal speed ranging from 5 to 400 µm·s^-1. As the withdrawal speed increases, the segregation of Si element intensifies, resulting in an increase in the area fraction of the Si-rich phase. In addition, the primary Al dendrites show significant refinement with an increase in the withdrawal speed. The relationship between the primary dendrite arm spacing (λ₁) and the thermal parameters of solidification is obtained: λ₁=127.3V^-0.31. Moreover, as the withdrawal speed increases from 5 to 400 µm·s^-1, the microhardness of the alloy increases from 90 HV to 151 HV. This is a combined effect of grain refinement and second-phase strengthening.